For you at school, work, stuck behind a firewal, and/or can't use/install an IM client, meebo.com will allow you to IM directly through your browser and stick it to The Man at the same time.
Works with the following: AIM, ICQ, MSN, Yahoo, Jabber, & GTalk
It is still in alpha mode, so issues might happen sometimes. The only emotional issues will be yours if you get all uppity about it crashing.
Thanks to Go Flock Yourself (the Anti-Web 2.0 Hype site) for posting about this cool thingy.