iChat only works on the AOL Instant Messenger network. With that substantial drawback it does allow some pretty cool video chat effects.
It’s amazing me to the level of “we’re going to lock you in” there is in Apple products. Some might argue that it’s a actually more of a “why would you want to leave?” thing- but seriously, even their chat archives are plists- their own type of proprietary archive files.
Kopete, the instant messenger in KDE, can chat via MSN, AOL, ICQ, GoogleTalk, Jabber, Yahoo, and probably more.
If you need to get your old chats exported out of iChat, take a look at Logorrhea.