It does the job Trillian only works on Windows, and only for 4 protocols. (It uses plugins to extend this very slightly.) It has a "Basic" version that is free to use, but limited in some functionality. If you want the full feature set, you need to pay.
Pidgin (www.pidgin.im/) does the job for 17 protocols and works in Windows, Mac OS X, and Linux. It is free to use, abuse, and modify. (There is no pay version.)