I believe that most of the current online pokemon MMORPG's are built upon the Elysium Source gaming engine. I've codded and mapped for a lot of other games built off this gaming engine before, and if I was able to get a hold of the pokemon mod source I was thinking that I'd make my own version for this site. Only problem is (as I said) I need the source since quite honestly I don't feel like making the mod myself. So if anyone knows, please let me know. Cheers.
I use to play on pokemon crater: http://www.pokemonbattlearena.net/ You collect, level and trade rare pokemon around with other people.
Pokemon World Online used to be open source, but I don't know if it still is. The game has come a long way since I last saw it and is offering premium subscriptions and stuff now, so...the likelihood of it still being open source is low, but you can check around. You could try 'mailing the owners, they might give you or offer to sell you the code.
@ retario, thoroughly useless advise. Arldey, I checked PWO out a couple weeks back and it's running closed source now. No way am I buying the source code either lol, I know someone out there an open source version is circulating. Just need to get my hands on it.