i use both ichat and a program called aMSN. aMSN allows you to send and receive video feeds through it from mac to pc and vice versa but no audio yet.
Adium also works great but i believe only supports txt with no video or audio at this time (i may be wrong not checked this for a while).
For my PC friends i use aMSN and MAC users i use iChat. iChat is also meant to support jabber networks which would include AOL IM so if your friends using MSN messenger switched to AOL IM then this should work with iChat.
Alternatively educate your friends in the error of their ways and switch em to a MAC!