Just started to play with Jogger - using Jabber to add comments to a blog.
Jabber is cool - it's Open Source, Open Standard ( look no patents!), can monitor AIM, ICQ, Yahoo and MSN, plus IRC etc. etc.
Lots of clients to choose from. And heaven for developers: add transports to other IM systems (Sametime anybody?) , write whole clients, add services to the server, run XML-RPC through firewals.