We've been using Trillian by Cerulean Studios for over six months now, and we really like it. It supports all of the major IM services in one program. All of your buddy lists are consolidated, and you can of course carry on multiple chats with multiple people over multiple services at the same time. We've found that it actually works better than some of the actual IM clients (hello, MSN?).
Some things we like:
- It unifies the interface for all services
- It looks like something you'd use for business (no dorky interface!)
- Extensive activity and chat history
- Automatic lookup of terms and words via Wikipedia
- Lots of customizable features