Many dating sites and social networks offer free IM and there are many more IM service providers and most view the online dating industry as a key vertical market. Here is the list of some of them.
AVChat
CometChat
FlashComs
Interactive Networks
Omnidate
Trillian
VisitorsCafe
Courtland Brooks has reviewed 4 top IM service providers in July 2009; toksta.com (dead now), ekko.tv (dead now), Meebo.com (dead now) and Omnidate (also seems to be dead now). You can download the Basic IM Analysis here.