This is a design I made for a spider only tower.
We designed this shortly after finding out lots of signs causes alot of lag in minecraft.
We made a Spider eye tower and a Strings/Wool tower with this design.
Spider Eye Tower was build shortly after xp was added to the game for making splash potions.
The Stings Tower was rebuild 2 weeks ago finished last week :)
I will upload vids of the towers in a few hours :)
But first here is the tutorial:

* Pistons activate the redstone and deactivates the pistons allowing water to push the spiders off.
* Be sure to hide the always burning redstone torches (Even the low light levels will reduce mob spawning a little).
* Because of this we use sticky pistons+ a block sice the light of the redstone can go thru a piston.
* Use glass above the pistons to make only spiders spawn.
* I've also included some designs we came up for other mobs using similar systems.