Another vote for Adium here as well. I used to use Proteus (version 2-4), but got tired of the problems with it and moved to Adium 0.80. Managed to keep all my history with a Proteus to Adium converter, and now since Adium keeps each chat session as a separate file, I can search for things via spotlight.

IRC, I use X-Chat Aqua. Works identically to X-Chat, but adds a few Aqua interface specific things.

Most multi IM programs on the Mac don't include IRC, or they do a poor job of it. Jabber conference room support in Adium isn't all that great either currently, but plans are in place to make it better for 0.90.