Adiam is an awesome instant messaging app for OS X which is based on the same code as Pidgin.  Adium has the advantage of being the same as many other open source instant messenger clients out there as well as being coded specifically for OS X and all the Aqua goodness one could ask for.

The latest release of Adium (v1.4) was let loose on 31st October this year and touts the following changes:

• Requires Mac OS X 10.5 or later
• Added Twitter
• Added IRC
• Greatly improved group chats and group chat bookmarks
• Updated to libpurple 2.6.6 with added fixes
• Mind-bogglingly large number of enhancements and bug fixes

The addition of IRC is one that I am really excited about.  For over a year I have used the OS X app called Colloquy and I really wish I could say it was good, but it really leaves a lot to be desired.  It's a fine IRC client, just a terrible attempt at creating an Aquafied interface.

For long time users of Adium, IRC feels natural, it feels like a group chat and it really works.  Throw in tabbed conversations and you can switch between IRC channels, MSN chats and other features.  It's a great addition to an already brilliant app.