I've used the free version Trillian for years, but I just switched to Miranda IM and couldn't be happier. It took a lot of work to find, install, and configure all of the plugins, so it's not for computer novices, but I've never seen such a customizable chat client. The initial install of the client sucks, but once you get the right plug-ins installed and configured, it is fantastic. I finally have XFire in the same chat client as AIM, MSN and ICQ, which is nice.
I tried Pidgin before installing Miranda, which was quite good, but there was no way (that I could find) to remove it from the taskbar when the buddy list is open, which was a deal breaker for me. For the average person I would probably recommend Pidgin, but for power users I recommend Miranda.
What does everyone else recommend and/or use?