yes the ones mentioned above are deadly, the i go you go...those are killer.
off the top of my head
1weighted pushups...
2. push up position, go down to half way, hold 10 seconds, down all the way hold 10 seconds, up to half way again hold 10 seonds, up 10 seconds, and so on. These static hold ones are deadly. try em with weights
3. put some phone books or someting on each hand under each arm.try to touch chest to ground, this is mainly for stretching, not really power performance, but if your more flexible it does work mirracles for your pushups.
3. An overall excerises that i found works really well is with a barbell, this is done standing up, take a relatively light weight and hold your arms out parrallel infront of you, then keeping the barbell parrallel extend your arms all the way out and back in, kinda like your throwing an arm punch. all the mean while keeping the barbell parallel, this works wonders for upper body work out, for me anyway.
4. santana t-pushups
good luck...
i'll post more if i can think of any...