If you're not impressed with Adium and you don't like chat clients such as Skype or MSN Messenger then you should take a look at Colloquy.
Colloquy is an advanced IRC, SILC & ICB client which aims to combine the best of all these messenger clients in what is a very elegant and refined interface. It supports everything you'd expect a decent client to do including AppleScript, file transfers, emoticons although unfortunately, it doesn't support either audio or video chats.
Probably the best part of the client is that fact that it can be upgraded with lots of cool plugins. These include such things as Transbot which translates to and from any language supported by Google's translation engine and Grab URL takes a URL in a chat room and attempts to replace its text with the page's title.
In all Colloquy is very interested and elegant chat client that would score much higher if it supported audio and video but as it is, it's a great chat only client.