You can also use it for Yahoo, MSN, AIM, and various other protocols. It's a great IM client and I hear that the next version will have video support.
For those using GNU/Linux or other non-MS operating systems, there's Pidgin. There's even a version that installs on Windows. Pidgin also supports multiple protocols including MySpaceIM. Adium actually uses libpurple, the library that is used for Pidgin, so think of Adium as Pidgin for Mac OS X.