http://en.bitcoinwiki.org/Egalitarian_Proof-of-Work_Algorithm talks about it in general and
http://en.bitcoinwiki.org/CryptoNight has a description of a specific one.
IMO, the only way you could truly have an ASIC-resistant cryptocurrency is to change between a large (or unlimited) number of entirely distinct algorithms, unpredictably, on the order of weeks (or whatever is fast enough to keep people from creating and running ASICs or FPGAs for it). Even if you do have a completely ASIC-immune algorithm, people can just build mining farms of CPUs and/or GPUs. You can't completely protect a proof-of-work-based cryptocurrency in this way.
"ASIC" just means "something made to be great at this algorithm". No matter what that algorithm is, if it's a single algorithm, you can build an ASIC for it. Some algorithms lend themselves better to ASICs (e.g. SHA256) than others (e.g. Scrypt), sure, but it's ridiculous to think that desktop CPUs will always be the best for some algorithm.
