I have contacts on spread across a number of different instant messaging platforms: Yahoo!, Jabber/Google Talk, AOL/iChat/ICQ and MSN services. I moved across different services as I moved jobs and clients, yet maintaining those contacts has a high value. Adium is an open source OS X software application that rolls all those messaging services in the one client.
Adium made it to v 1.0 the other day and I updated to the latest version. It has a number of features and a new look and feel that give it feature parity with many of the standalone clients for MacOS X, but without the usual advertising tie-ins. Adium has a whole eco-system of add-ons including a scripting language and a range of themes so that you have a range of look-and-feel options for IM conversations.
Some of the better ideas that have been adopted by latest version of mainstream messaging clients were out in this add-on community for a good while (like your current track playing on iTunes in your status field) and have now been incorporated into the main Adium product.
Organisation of different accounts is really easy and there is a more professional sheen to the latest version. However, the best thing about Adium is the price, free!