SplitTweet at http://splitweet.com is a great tool if you have a number of accounts on Twitter. Rather than switching between them from time to time in order to Tweet, you can set an account up her for free and bring all of your accounts under the one roof.
Splitweet will display the Twitter feed from all your accounts at once, or you can turn individual accounts off and on. When actually tweeting, you can pick an account to tweet to, or you can do a broadcast message across all your accounts at once.
Some very flash technology this and very useful for handling multiple accounts

Friendfeed at http://www.friendfeed.com is an automating tool you can use in conjunction with Twitter. Friendfeed is a bit of a hub if you like where you can set up links to all your other accounts. Friendfeed collects information from those accounts and publishes it in your Friendfeed feed.
Although Friendfeed is a topic almost in itself, I mention it here particularly because of it's ability to link your Twitter and Facebook accounts among others. If you have a Facebook account and you link it and your Twitter accounts in Friendfeed, then what will happen is that all your posts in Twitter will be posted on your wall at Facebook. It means you are running two social media accounts via one and saves you time