* [http://mibbit.com/ Mibbit]: is a great online chat client that doesn't require any installation and setting up an account is free.
* [https://addons.mozilla.org/en-US/firefox/addon/chatzilla/ Chatzilla] is an extension which may be installed in add-on for Firefox; it comes built-in with SeaMonkey. You can also install it [http://chatzilla.rdmsoft.com/xulrunner/ standalone].
* [http://colloquy.info/ Colloquy], a standalone Mac client with a nice looking interface. It also has a mobile client for iPad/iPhone/iTouch.