Multi-service Chat applications

Useful for chatting online, because they handle multiple services (AIM, Yahoo, MSN, ICQ, etc all are different. A few years ago you had to use separate applications to access each one - not so much anymore)

For Windows: I like Pidgin, but Trillian is also good.
For Macs: Fire

All of these are free. Trillian does have a "Pro" version with additional features.