Simkl came out of private beta and opened up to the world as a publc beta this week. Simkl History Saver saves all of your IM conversations, maintains a backup of contacts, and provides a search interface for your chat history. The service is compatible with all of the major chat clients (48 in total) out there, including iChat for Mac OS X, AIM, ICQ, MSN, Yahoo, Trillian, and Adium. Simkl has been testing in private beta since December 2007 with more than 10,000 users.
Otis Collier has posted this demo of Simkl on YouTube (he is not associated with the project).