If you use Twitter for more than just messaging - for example, tracking expenditures or appointments - you may want to take a look at Twistory. It's a handy web tool that creates a calendar feed for your past status updates.

Simply enter your Twitter username where requested, and Twistory creates a feed that you can subscribe to in a compatible calendar application like iCal, Windows Calendar, Outlook, or Google Calendar.

Want to track multiple accounts? No problem. There's no limit to the number of feeds you can create with Twistory. It's also a good way to archive tweets from other users you follow - say, for example, @downloadsquad.