Twistory is yet another free tool to back up online accounts, specifically Twitter posts. It goes beyond a simple data dump, though, by plotting your tweets in an easy-to-import iCal feed.
Twistory is really simple to use, and doesn't require authentication. Just feed it your Twitter username—or any Twitter name, really—and it kicks back an iCal feed link that you can push into iCal, Google Calendar, or any calendar app that accepts the .ics format. Start using it with Google Calendar before you reach Twitter's 3200 tweet backup limit, and you'll have a search-ready, exportable database of your tweets.
Twistory is a free service, doesn't require a sign-up to use. Calendar geeks, feel free to share any neat hacks you can pull off with tweets in an iCal feed in the comments.