Do a forum search. There are tons of theses threads already.
You might also try this website.
Just make sure whatever you download is UB. Or it will run really slow or not at all on your iMac.
Okay, I'll third Adium, especially if you're not going to be doing any video chatting and you enjoy customizing your chat program. I'd also recommend Fugu for GUI based SSH/SFTP stuff, and SubEthaEdit for text editing. Really though, the www.opensourcemac.org website has done a good job picking most of the open source apps that you may find useful.