Most all of the major instant messengers have web-based interfaces. As long as you have Java installed they should work fine.
Here are the links for the "big guys".

https://my.screenname.aol.com/_cqr/login/login.psp?mcState=initialized&siteId=ae40_prod_aim&seamless=novl
http://login.yahoo.com/config/login_verify2?.src=pg&.done=http://messenger.yahoo.com/webmsgr/launcher.php?back=http://messenger.yahoo.com/
http://www.icq.com/download/icq2go/
I haven't found a web-based interface for MSN Messenger. I'd be interested to know if anyone else has.