Protoss are the most powerful race in the mid game (some think that zerg are but, I think Protoss have the slight edge), mostly because of the powerful rushes like dark templar or reaver. Sometimes a rush with zealots only can win you a game here. You can also decide not to rush and get advanced units faster, like high templars with psi-storm, corsairs or you even carriers but with sacrificing a lot of economy for teching up.
Dark Templar Rush
Dark templars are the only permanently cloaked attacking units in the game (well, every unit in starcraft is unique, but that's not what I had in mind :). You can often catch your opponent off guard and/or rushing you with melle units when you have around 2 dark templars. By using it properly, you can severely cripple your opponent until he gets some detectors.
1. Do the initial build order,
2. Use the probe that warped in the gateway to build an assimilator,
3. Build two more probes and a cybernetics bay,
4. About the same time, the assimilator will finish, so put 3 probes on it,
5. As the cybernetics bay finishes, build a citadel of adun,
6. When the citadel warps in, first build 2nd gateway, templar archives and another pylon.
As soon as you can, start cranking dark templars. This build order can be used against zerg, but just for taking the map control. If you think you can be rushed with dark templars yourself, build an forge after the assimilator and two cannons (one when the forge finishes, and the other after you start building dark templars).
The Fast Reaver Drop
It is a fact that you can drop any unit in your enemy's base, but none so useful as the reaver. The fastest reaver drop consists of dropping one reaver and 2 zealots.
1. As always, do the initial build-up,
2. Warp a assimilator,
3. Build two probes and cybernetics bay,
4. As soon as you gather 200 gas build a robotics bay,
5. Build 2 pylons and 2 zealots,
6. After the robotics bay has finished, start building a shuttle, then build a support bay,
7. When the support bay finishes, build an reaver and drop.
The reaver drop is particularly useful against Terran, then Protoss, and sometimes Zerg.
The Powerful Reaver Drop
The second version of the reaver drop would be dropping 2 or 4 reavers. You can notice that one robotic's won't do now, but you'll need two (if not more).
1. So, after the initial build order, warp the assimilator,
2. Build two probes and cybernetics bay,
3. When you gather 200 gas build a robotics bay,
4. Build 2 pylons and 2 zealots (if it is a island map, don't build any zealots),
5. Shortly, you'll have another 200 gas, so build the 2nd robotics,
6. When the first bay finishes, build a shuttle, then a support bay.
When you have enough resources, build 2 reavers, one more shuttle and 2 more reavers and drop. If your opponent has no air units by then, he's as good as dead.
Zealot / Dragoon Rush
In the midst of the game, protoss have the possibility to make one of the best rushes. Zealots and dragoons are kicking combo, as you will see if you use this strat.
1. After the main build-up, it's important to remember cranking probes up until you get to number that fits your minerals.
2. Build 2nd gate as soon as you have 150 minerals, and always build zealots in both gates,
3. When they are finished, warp in an assimilator and the 3rd gateway (you probably won't be able to pylon support this much gates properly, so my advice is to get a pylon whenever you have 10 control left).
4. When you have 100 gas build a cybernetics bay and when it's finished start cranking dragoons.
When you feel you have enough units, attack. This build order leaves you without detection, so the forge would be a proper thing to build next.