From Drewbie of Root-gaming.com
pump marauders + concussive shells asap
add 2nd + 3rd rax when you have enough money
3rd depot after 3rd rax
upgrade stim when you have enough gas
make 1 techlab + 1 reactor when your 2nd/3rd barracks finish
combat shields as soon as 2nd tech lab finishes
factory when you have enough money ~ around 35ish supply
when factory finishes add starport asap + reactor on factory
switch reactor onto starport and pump medivacs non-stop
this build is really safe vs any all-in protoss build, and if they are greedy and try to expand too fast, you can punish them with drop harass or just rolling them over. Basically, they have to sit in their base and defend for 10 minutes.
Make sure your scouting is good, go out in the map with your first marine + marauder and chase away any probes, the protoss won't be able to come out because of concussive shells. They also won't know if you are going 3rax or fast expoing. Try not to use any scans unless you have absolutely no idea what they are doing. I like to put 1scv or 1 marine behind the minerals at their expo, so I can see when the P expos. Also if your scouting scv got no information, build a reaper and sneak it into their base, it doesn't matter if you kill any probes or not, just use it to scout their tech path.
If you see 4 warp gate, just make 1-2 bunkers and wait for medivacs to come out, then you can drop in their main + roll out the front once medivacs are out.
If you see a robo, they are playing pretty safe + standard, and won't be able to move out, keep close tabs on what they are making, if they expand too fast you can kill them. If you the P is going immortal heavy, build lots of marines and try to harass / fake attack and kite his units. Just make sure not to get caught off guard by colossus, if you see a robo bay making, you should be able to drop in the back + attack the front and roll over the p before they get 2 colossus/ the range upgrade. If you don't scout it in time just expand and build a couple bunkers and make vikings. Vikings + bunkers are pretty invincible vs colossus/stalker/zealot. Also drop their main when they move out with colossus ^_^.
If you don't see 4warpgate or a robotics with your reaper scout, and your scv/marine at their expo doesn't see an expo, then the p will be going void ray or DT, so add an engineering bay ( start upgrading +1 attack asap) before your factory and build 1 turret at your ramp and 1 at your mineral line. It's a bit expensive to get the engineering bay + medvacs out and they will probably be going voidray, so pump almost all marines, and theres no way you can die to the p's timing attack. Once you stop their timing attack, the game is basically over, you can just wait for 2-4 medvacs and +1 attack and roll over the p. If you took some damage and don't have enough units to counter and win the game, then you can just expand and try drop harassing, and just try to deny the p an expo.
This build is really powerful in the early game, but I would suggest a transition into tanks or ghosts or banshees or something if the game goes long, mass bio is pretty useless once the P gets 3 base, but you should have an economic advantage and be able to make some stronger units if you use your units wisely.