One of the most common Zerg tactics is the simple mass Hydralisk assault. It usually works, because Hydras are excellent units for their build cost and supply use. However, the Protoss can turn back a Hydra swarm by building a line of photon cannons, one or two deep, with Reavers lined up behind. Make sure you have pylons interspersed between your cannons, so that you don't suddenly have several cannons unpowered after one pylon is destroyed. It's a good idea to put a tag on your Reavers (ctrl #) so that you can keep building scarabs easily. Upgrading scarab capacity is nice, too, if you need to. Don't bother to upgrade damage unless you have extra money, or your opponent is using Ultras on the front line. The other main Zerg unit for a big attack is Guardians, because they're excellent base destroyers. Guardians are usually employed above the Hydras, because they have no air attack. However, if you're using your reavers properly, the Hydras should be bursting like big zits all over the place, and the guardians will be vulnerable to your air units. If his guardians get close enough to attack the Reavers, they get nailed by photons. If he's sending lots of guardians, have a few scouts come at them from behind, so his hydras will have to move to attack your ships.
If your defense is buckling, let the reavers and cannons do it alone and just send air units at his base. SOme players will panic and break off the attack to defend. If they do, engage him outside the base, and then reaver drop him.
This is a costly strat, but the other guy would have to spend tons to make enough hydras/guardians to overwhelm you. GL, HF!

CreepColony's Opinion: Cannons aren't very effective against mass hydras. Use reavers and corsairs to disruption web the hydras. Corsairs and dragoons are also effective against guardians.