Tweet Later: http://www.tweetlater.com/84911.html — Allows you to post tweets to be sent later as well as to send automatic messages every time someone new follows you? Users can set up Tweets to happen later on in the day, week, or month. Also allows you to set up Twitter alerts for any keywords, people, companies, etc. that you want to track. Alerts are sent to you via email. Also allows you to auto follow those who follow you and send them a welcome message. Finally, if you have multiple Twitter accounts, it allows you to tweet to one or more accounts at the same time (I use this to tweet quotes I like to more than one account).
Their free service allows you to do all the above. Their new professional service allows you to direct message all your followers, integrates Ping.fm, schedule @replies and direct messages, and read and post to all your Twitter accounts at once.
Neat service. I use it every day. TweetLater.com makes life on Twitter a lot easier.