Before we discuss counter builds, let's talk about what an early Zealot rush is:
"my defenders got wiped by 4-5 zealots and more and more zealots kept pouring in and eventually killed me"
This is the dreaded 2 Gate rush.
Your first goal is going to be identifying it. Luckily this is fairly easy to do. If you scout with your 12-13th drone/probe/scv you should get to your opponents base while his first Gateway is still building, worst case while his second is. All 2 Gate rushes start the same: Pylon, Gateway, Gateway, Pylon. This makes them very easy to pin down. If you see that second Gateway go up before the Cybernetics Core you know he's playing some sort of rush (pro tip: you can click on a Protoss building warping in to see what it will be). If he does a delayed rush, chances are you'll already be in a better position to defend, so you really want to watch out for this early rush.
The attack that will come should be 5 Zealots with 2 more in route (obviously he may wait for more or less, but this is the best time for him to attack).
As Zerg a 2 Gate opening is one of the scariest things to deal with (right after Hellion harass into Mech). If you fast expanded, use the expo as a damage soak and cancel it before it gets killed (if you can). Ideally you'll want to keep that expansion alive as his economy will already be devastated. When you scout a 2 Gate, you'll want to build a Spine Crawler almost immediately. Additionally, a Roach Warren is also recommended. Roaches are very strong vs Zealots (slow attack and starting armor vs a unit with fast attack) and if you can position some by your ramp or other choke you can cut the surface area of your Roaches in half. If you're already into a more speedling based build, then you're going to want to wait for him to surround your Hatchery or engage your Spine Crawler. This will naturally give you a very nice surround on his units as they will spread out. I also recommend bringing your Queen along for ranged dps (and to infuse the Spine Crawler).
The key thing for Zerg about repelling a 2 Gate play is the recovery. Early 2 Gate is so effective that even though he is putting himself at an economic disadvantage, he may do enough damage to your economy to win. You'll want to do the following: Put an Zergling outside his based (this is a scout for his next move); Tech to Roach or Hydra or Muta, as 2 Gate will likely lead into 3 Gate, 3 Gate + Robo or 4 Gate; Power drones, this one is key as he's making a huge economic sacrifice to push with 2 Gate and you need to exploit this for victory.
Terran traditionally have less problems with 2 Gate as they can usually wall off before the Zealots get there. If you already have Vespean Gas then its ok to go for Marauders with Concussive Shell, however, if your strategy didn't include that, you're going to want to skip add-ons and throw down a second Barracks when you scout the second Gateway. With Marines faster build time, you can create a tough Marine force behind your wall well before his Zealots arrive. This ruins the fact that he made an early sacrifice to go for a Zealot push and should give you command of the match. From here is very easy to transition into an MMM or even BioMech which should give you a quick victory.
As Protoss you're going to also want to wall, stick a Zealot or two in the opening and push for Stalkers. If you can have enough 1 Stalker out and your opening plugged before he gets there, chances are you've already won. It is not unusual, upon seeing a 2 Gate, to throw down a second gateway yourself. As defender you'll naturally have the advantage as his troops have a longer rally distance, and you can control the choke. If you push out an early Sentry, he'll probably Rage Quit and call you a cheater.
Now if you're dealing with a Proxy Gate (which is different than a 2 Gate) you have slightly different things to watch for. Lack of Assimilators and Pylons in his base are your best clue. At this point you'll want to start taking your scout and covering all the corners of the map. Additionally move a scout to outside your natural or the nearest Xel Naga tower.
First off, you should be getting Stim before Concussive, I'll explain why in a second.
The Marauder is both a vastly overrated and underrated unit. It has pitiful dps (against anything that isn't armored) and largely exists as a meat shield for the much higher dps, much lower hp, Marines. The ideal Marine/Marauder ball is relying on Marauders to tank while Marines dish the damage. Let's take a closer look at the Marauder:
125 HP
1 Armor
10+10 damage (or 6.67+6.67 dps)
Against your average Zealot that 1 armor is going to cost him ~15% of his damage, while the much more cost effective Marines dps him down. Additionally, any Stalkers only do 70% as much dps (as the Marauder).
So Marine/Marauder is already set up nicely against Zealot/Stalker, which brings me to Stim. Marines, being your real dps here, really need Stim to help take out the slower Protoss units and it also gives them the ability to kite. Compared to that Concussive Shell is just icing. In fact, you'll see a lot of Terran (like Jinro) move out just as Stim finishes and Concussive starts, because they want to maximize the timing window Stim represents (and because Concussive's shorter build time often means it'll finish just as they start engaging).
Now you asked about earlier pushes than Stim, and the list is very short:
Both of these plays fail against a 1 basing Protoss player. The truth is, Stim is very early, and very powerful, you're not going to find anything better before it. If you're consistently losing a Marauder heavy ball while dealing early pressure, then I suggest you reexamine your composition. Most 3 Rax play has an even amount of Marauders and Marines. Earlier pushes (like 2 Rax), tend to be very Marine heavy, with a few Marauders thrown in to act as meat shields.
I hope this helps
Best Answer
As you reach higher levels of play, if scouted properly, the defender will have enough units that will hold off the reapers from damaging their harvester lines. Granted if they don't catch it, it can do massive damage to their economy as the reapers have a +damage versus light units (which all harvesters are).
In gold or below leagues, the reaper rushes can still do a lot of damage, but if you watch any of the higher level matches (most can be found on HDStarcraft or HuskyStarcraft's youtube channel) most high level players will use the reaper to scout and try to harass (they might get 2 or 3 harvester kills), but it doesn't do much else more than that except the very rare occasions with good micro.
It's also very situational as well, against early zerg yes you can kite zerglings with no speed, but once they have the speed boost, reapers are surrounded and chewed through. Against protoss, you can kite the first zealot they put out, but by the time the zealot is dead, a stalker will almost be done and that stalker will kill the reaper. Against other terrans, if you go for their SCVs, the marine will kill you.
Personally I sometimes use it against either zerg or protoss, never against other terrans.