I haven't seen this one in the archives. Most of the readers probably know that iChat keeps conversation logs [robg adds: it's an option in the Messages section of the prefs which is turned off by default].
However, did you know that you can go to the ~/Documents -> iChats folder and drag conversations to an e-mail to send to other people? Useful for reminding someone what was previously discussed. They can open them in their iChat application.
[robg adds: The iChat logs are binary files, so (somewhat annoyingly) you can't browse them with a text editor. You can use grep -l "some_text" *.chat in the Terminal to list which chats contain certain text, but you'll need to use iChat to then browse that conversation for the text you were looking for ... but you cannot then use Command-F within iChat - argh! So you have to select all then paste into a text editor. The chat logs would be much more useful with a better searching mechanism...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030219083335168