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Use Quassel, open source and cross-platform.

I second quassel. Not only is it an excellent client, it has an awesome core/client architecture.

I use Pidgin. I also use it for XMPP (Google) and private Ejabberd and some other protocol I forgot. Also use the same on Linux.

Well, if you don't mind switching your operating system to Emacs, there's ERC, and if you are willing to run a WeeChat instance on a *nix/Mac/Win+Cygwin machine, you can connect to it with weechat.el. I recently switched from the former to the latter :).

I used to use mIRC a lot, fully scriptable/customizable. Best client on windows Imho.

ahhh, the good old mIRC!

You can do some amazing scripting stuff and really customize it.

There are a lot of custom versions of mIRC online with crazy color themes and what not.

http://hawkee.com/mirc/scripts/

I prefer KVIrc. Very customizable.