I'm very often executing the following build in platinum/diamond league games (EU server):
SCV Supply Depot Refinery Barracks Saturate the refinery fully Barracks Barracks Reaper Supply depot 3xReaper ...into... Command Center or Factory, then Command Center
I can measure the execution by the moment, when the last reaper is queued. If it's around 2:01, it's totally perfect, 2:04-2:06 is OK, 2:11 is accetable, later - it becomes non-efficient.
Note, that I mostly play on 2v2-3v3s, which may be an important thing.
This build is designed to punish greedy players, who start with base-first. 4 reapers are enough to take out a Queen, Stalker and easily a bunch of Marines. They seem enough even to take out a Siege Tank if it has no support or is supported only by SCVs. Marines, low-speed zerglings and no-charge Zealots stand no chance over such a small army. Not to say, that from what I remember, 4 reapers one-shots workers.
The advantage of such an assault is that it may be performed head-on. Even if you don't sneak Reapers in the mineral line by cliffs, you can simply engage forces and very often leave such a battle without losing a single reaper. If you then manage to produce additional reapers to increase your army to 6 or 8 of them, this is ususally a game-over.
Now the interesting thing is counter for such early one-base all-in cheese. Firstly, static defenses totally kills Reapers. One Spine Crawler, Photon Cannon or Bunker built near mineral line ends up this atack and forces me to go to mid-game transition (most likely to bio, having 3 barracks built already). However, static defenses are rarely seen in pro-player matches, because they consume resources and are - well - static, so immobile.
Also, this build is highly effective against Terrans and Protoss, but not-so-much against Zerg, because Queen - though possible to kill with 4 reapers - takes a lot of time, which gives Zerg oportunity to, say, queue a Spine Crawler or two (one is not enough, because it can be killed by those reapers before hatching).
You can take a look at pro-player matches TvX - just check, what their opponents have at around 2:50-3:00. This is the moment when 4 reapers run into the base. Even having 1 reaper some of the pro players manage to kill a Zergling, when Zerg have a Queen and 4 Zerglings - now imagine, what could have happened if there were 4 reapers instead of 1.
It is also not effective against non-greedy players, who stands on one base longer, because at that point they usually already have like 2 Stalkers, a bunch of Zerglings (and Zergling Speed already queued up) or a sieged Tank, which shuts down such an attack completely.
One more thing - this requires enormous amounts of micro to perform properly, because you have to babysit Reapers very carefully. This leaves me - as a definitely non-pro player - often in situation when I have CC, 3 barracks, a lot of money and no defences, when opponents retaliate. I believe though, that on pro-level players are able to micro/macro this scenario properly.
I actually executed this build successfully numerous times on 1v1s as well, though in Gold League.
So the question: why such Reaper-heavy opener is not executed on top-tier matches? We see zergling push every now and then, massing Adepts at the early stages, but Terrans barely ever use full potential of Reapers and their grenades.