Basically, any coin that uses an algorithm for which there is no ASICs yet, is asic-resistant.
Some algorithms are made specifically in a way that makes building ASICs for them very hard/costly.
There's ASICs for SHA256 and Scrypt. And rumors circulating about X11...
Don't even try mining an ASIC'ed algo with a CPU/GPU, you'd just be burning money.

Any other algorithm, and you have a fighting chance.