UPDATE 2/18/12 - 4 Hellion Opener into siege timing attack TvZ (LKBO)

I was having a hard time beating Zerg with Terran so I reached out to the viewers in my chat and LKPL shared how he likes opening TvZ with 4 hellions into a 2 tank timing attack.  This strategy is best used when the Zerg player goes Hatch first.  If he doesn't expand by the time you've upgraded to an orbital stop what you're doing and get ready for a 7 roach rush.  My first indication that this may not be the right way to go is when I see he's started his spawning pool before he expands. EDIT: This build can work vs the 7 roach rush, just don't sacrafice your hellions, get them behind the wall, pull SCVs and keep producing marines from your barracks, siege tanks from your factory, and try to keep everything alive as long as you can.  Sacrifice SCVs first if anything.  A little micro will save you from the roach rush, but it's not easy... I have edit'd the build order below to reflect my current build.

The build goes as follows:

10 Depot @wall - > scout

12 Barracks (@wall, finish wall with 2nd Depot when Barracks 100%) -> 1 Marine

13 Gas

15 Orbital Command

@100 gas build a Factory and take your second Gas

@50 gas build a reactor on barracks

Flip the Factory to the reactor and build a tech lab on your barracks.
@400 minerals BUILD YOUR EXPO IN YOUR BASE
4xHellions from your factory*
train stimpack at your barrack's tech lab
land your factory on a new tech lab, train siege mode, and build a barracks on the vacant reactor
build a 3rd barracks w/tech lab and starport w/reactor
upgrade your expo to OC and fly to natural while building 2xbunkers at natural choke

*You MUST keep these 4 hellions alive unless you can kill 10(TEN) drones.  They give you a huge advantage in map control and force units from the zerg player.  They are your answer to the random lings spread all over the map.  They force the zerg player to stay at home, they deny creep spread, they keep the zerg off balance... they are VERY VALUABLE.  Don't throw them away.  If you run into a well defended expo (multiple spine crawlers, queens, roaches, etc) then deny creep, go to the third and park your hellions where they'll burn any drone coming to drop a hatch, etc.  Be as active with these hellions as your own personal control will allow.  I tend to back them off to a safe distance to prevent creep spread for a minute or two and then just park them at the 3rd.  Of course, if you see a weak spot, burn drones and then deny mining opportunities as long as possible.  Also, most zerg mains don't have spines, so if you can run by to the main... go for it.  Just don't do it if you lack confidence in the fact that you'll kill 10 workers.

At this point you're training siege mode, stim, and combat shields.  These are the most important.  If you have the minerals/gas then build a couple engineering bays and train 1/1.

Produce siege tanks from the factory, marines from all barracks, SCVs from both OCs, supply depots, and medivacs from the starport.  When you get your 3rd siege tank w/siege mode it's time to move out.  This push comes around the 10 minute mark.  This is also the time the first mutalisks will hit the field, so put up a turret or two as you're moving out to prevent free SCV kills.  NOTE: if you time this right you'll be at his natural when the mutas pop and they won't be able to leave.

My army typically looks like 20 marines (w/combat sheilds, stim, 1/1), 3 to 4 siege tanks, however many hellions survived, and 2 medivacs.  Any reinforcements are rallied to my natural to prevent any unwanted nonsense from going on while I'm out having fun.  If I need them I'll scoop them up as necessary.  If you've prevented creep spread and been annoying with your hellions you've set yourself up for a nasty timing attack.

When your siege tanks are in range of the spines/hatch siege them up, get in range with your marines, and go to town.  I typically try to hold stim for his reaction.  If it's banes, stim and run past your siege tanks.  If he doesn't change targets and go for the tanks you won't take a single loss.  There's no way he has enough banes at this point to kill the tanks and your bio.  He has to pick the tanks or lose them all for nothing.  If he comes with mass lings, just get back to your siege tanks, stim and murder ALL the things.  Mutas are the best because there's no way he can handle 20 marines with 8 mutas + light lings.  Just don't let him pick off the tanks for free.  If he builds roaches in response to your hellions he'll learn really quickly that combat shield and stimpack on marines with medivac support and 1/1 are a strong counter to un-upgraded roaches.  If he doesn't react, use your medivacs for vision and let the siege tanks do work on static defenses.  Prioritize queens (no more larva), drones (no more cash), then tech (no more army).

Once you feel confident you won't need to split your bio or move your tanks for a second go and scoop up your army from your base and A-move them to your current army.  If he still hasn't quit, build a 3rd at your natural, fly that to your 3rd, and upgrade to a planetary fortress.

Pros and Cons of this build

Pros:

• Keeps the zerg player off balance early game w/hellions.
• Stops lings from gaining map awareness.  
• Keeps zerg forces at home
• Encourages the zerg player to do things that are weak to this build (mutas for harass, roaches to counter hellions, spines)
• Hits the zerg natural as the first Mutas arrive.  There won't be enough.  There's no way he'll be able to repel your attack with even 12 mutas + a few lings queens and spines.  
• Strong macro with early expand + early 3rd
• Great map presence!  Take the fight to his side of the map.

Cons:

• Weak to 1 base play (roach/bane all ins)
• Weak to extremely greedy play if your attack comes any later than 10 minutes (ex. double expo first into mass muta.  You can handle 10 mutas at 10 minutes no problem, but make that 20 and you're in a world of hurt)
• You are relying on upgraded marines, so banelings on creep can be a disaster if they are in proper numbers.

General Tips for TvZ

• Don't wait until you need to split your forces to split your forces.  If you have a ball of marines standing around waiting to do something split them up evenly around the space they are in.  It would be nice if we all could handle 300 actions per minute, but if we can't we can still survive banes with proper foresight.
• Turrets... lots of them... When you move out you are putting your base at risk.  A very very early spire can mean mutas in your base at 10 minutes.  If you wait until the mutas arrive to build turrets you'll end up having to turn around and go back home to clean them up.  Two to four turrets at every base (and around all production facilities... not just your mineral line) WILL turn a small muta ball around.
• If you lose your army in the push and don't cripple him outright his next response will be mass lings to clean up your base.  A few bunkers should be able to turn them around.  If you can build depot walls with bunkers behind (or in) the wall you'll be perfectly safe without sacrificing anything but 25 minerals per bunker.
• SAVE YOUR STIMPACK.  Don't blow stim to do extra damage until you know for sure what you're facing.  If he comes in with banes you want to be able to stim and out run them past your siege tanks.  Over stimming is suicide.  You'll only have 2 medivacs, so don't expect them to heal stim damage too quickly.
• Rally your medivacs to a medivac.  Zerg T1 units can't look up, so you don't really have to worry about medivacs being picked off on the rally unless he has mutas on the field.  If you're moving between 8:30 and 9:00 then you shouldn't have a problem getting 2 extra medivacs to the fight just in time for the engagement.  This goes for Vikings too.  They are easy to "misplace" on the map if you don't have a good rally point.  I really like rallying air to air in TvZ.
• Hold the Xel'Naga watch towers.  Seriously... send something there and know that he'll be sending 4 lings to kill that thing... so choose a thing that can kill 4 lings.  Nothing is worse than watching a replay and seeing that he starts morphing banes the second you move out because he saw it coming and having that cost you the game.
• PRIMARY THE BANES.  If you surprise him with the attack look for morphing banes near his natural.  If you can catch them morphing in hit that STIMPACK and kill them before they kill you.  Put your siege tanks on a separate hotkey and focus those punks from range while stimming behind the tanks with your bio.  This will save your life.
• Upgrade early and often.  3/3 marines are BRUTAL.  They can take on mutas in just about even numbers, they melt bases in small numbers, they are easy to remass and they are cheap.  If you're going for 3/3 at the 15 minute mark you're golden.
• Build more barracks and add attachments when you can.  Even if you're maxed you need to be ready to re-max at a moment's notice.  Remember, once zerg is maxed he's just banking larva.  You can keep up, but you better be prepared early with tons of production facilities.  
• Drop EVERYWHERE.  If the first push fails you're going to need to stay on top of the zerg or he will just expo to every mineral patch on the map.  16 marines and 2 medivacs mean load up and drop.  Don't throw your units away, but don't be afraid to mix it up.  Marines with medivac support can pretty much kill lings 4 to 1.  Just as Dark Templar are the answer to a Protoss who's behind, mass expo is the answer for a "from behind" zerg.  Why?  Because it works.  Don't let it happen.
• After the 10 minute mark be ready for burrowed banes.  Scan in front of your army as you move or bring a raven.  
• If the zerg player has exceptional creep spread build a raven.  Seriously... do it.  Have your raven and 10 or so marines clean up all the creep tumors you can and take back map control.