- Google Talk (my default. It integrates seamlessly with my Gmail account. Also, the only open standard and the only no-frills approach, which i like)
- MSN Messenger (most people I know use it, so there is no way to avoid it)
- Skype (used to be fun, SkypeOut is cool, if and when it works)
- ICQ (someone i know still uses it)
- Yahoo Messenger (Ok, i have an account)
I know there are clients like Trillian or Gaim, but somehow i haven't taken to them. It's still an account mess and not all features of all the messengers are supported.