Simkl: instant message archiving now open to public
If the thought of a third party (besides the actual service provider) archiving your instant messages doesn't bother you, you can try out the service for a week for free. The history viewer is web-based, and you can do full-text searches on your IM history.
Simkl supports most of the popular IM clients including Yahoo, MSN, AIM, Pidgin, iChat, ICQ, Trillian, and Adium. Keep in mind that many of these IM clients like Pidgin and Trillian can log your chats individually, but if you use multiple computers with different IM clients, Simkl can unify your IM logging.