Here are 5 Twitter based websites that are useful in doing stuff like searching for tweets to scheduling tweets :
1. Tweetscan : This helps in searching for tweets related to topics of your interest, similar to like a search engine for Twitter.
2.Twitter Answers : Ask questions on a specific topic and get the answers just like Yahoo! Answers. This needs a free signup to Mosio account and once registered, you can get upto 4 answers directly via IM in Twitter. Works for mobile phones too.
3. TwitResponse : This is a scheduling service that allows for posting of Tweets at a scheduled date based on preferences.
4. TwitterVision : It is like Google Maps for Twitter, displays the geographic locations from where tweets originate.The beta of this service can be found here.
5. Remember The Milk : This is a task reminder service for Twitter which will send you Twitter DMs (direct messsage) based on tasks that need to be reminded of.
[ In case you need to know about how to use Twitter commands, do check out this earlier post. ]