Backtype is a new tool that aggregates comments posted throughout the blogosphere and makes them searchable by keywords or commenters' names.
Other useful tools:
YackTrack -- another tool to search comments
IRSeeK: search conversations on IRC
Boardtracker, Boardreader and Omigli -- search discussion boards and forums
Mail Archive, MarkMail -- search email list archives (mostly older ones)
Copernic -- track changes on sites
A review of image search engines
TalkDigger -- tracks "conversations", not sure how well
Custom Google-based engine for many social bookmarking sites under one roof
Twitter and FriendFeed searches, of course
I also like searching the Raves and Rants section on Craigslist.