This build is pretty dialed in as far as time goes. Don't miss your timings! I'll put time up for the major events.
Send scouting worker immediately (literally 2 seconds into the game, right after you start building your first SCV)
6 - scv
7 - scv
8 - scv
:54 9 - DEPOT with the PROXY scv, do NOT pull workers to build this, use the proxy scv and build it the instant you have 100 minerals - this will become your proxy location
1:26 9 - barracks (with proxy SCV)
1:37 9 - Refinery
9 - SCV
10 - SCV
@ refinery completion, put 3 on gas IMMEDIATELY
2:38 build first reaper
2:34 13 build barracks #2 at proxy
3:07 BUILD ORBITAL COMMAND RIGHT NOW! Timing is critical on this one.
13 Reaper #2
15 Reaper #3
17 Reaper #4
3:40 Orbital finished, immediately drop a mule. Send another SCV forward to the proxy and get barracks #3 and #4 started ASAP without slowing down reaper production.
Begin harassment immediately upon your first reaper completion. He can easily take down a marine or harass mineral line/builders if there's no marines out yet, but be STUPID careful not to lose him. You need them to stack up and survive for this build to work best. He'll possibly have 1 marine out, use this time to try and kill it but do not let him surround your reaper with scvs/kill it. Speed is your advantage.
You can technically get out the first and fourth reaper about 4 seconds earlier if you do a build without an orbital command, but this build puts the proxies closer to the enemy base, effectively making the attack times (units inside enemy base) a hair faster while still allowing you an orbital command for really nice followup.
From here, you can keep pumping reapers non-stop to 8 total, just don't miss your mules and don't forget to build depots at home for supply. Add rax 3-5 at the proxy (send another SCV and keep building them) to continue the push, or grab your second refinery and start a factory. Keep dumping mules on your mineral line and cash won't be a problem. You can add a few more SCVs now too, no reason not to.
Usually they are dead by now, if you killed the first marine things really snowball from there.
It's damn hard to stop. Works decently well vs Z if you can keep up with the micro/macro during the attack.