1. TweetDeck
Desktop program for a dashboard view of your Twitter universe. You can post (and automatically shorten URLs) from TweetDeck. You can create custom groups of Twitter users you follow so you can see tweets from professional versus personal follows in different panes.
Supports both Mac OSX and Windows (XP/Vista).
2. bit.ly
URL shortener that can post directly to Twitter. If you share a lot of links on Twitter, bit.ly makes it a fast process by letting you shorten the URL and post to Twitter within the same screen. You can post to multiple Twitter accounts using one bit.ly account.
Extra helpful feature: bit.ly also keeps track of how often your shortened URL gets clicked.
3. TweetLater
TweetLater lets you schedule Twitter posts for the future. You can also have TweetLater auto-follow Twitter users who follow you.
4. TweetBeep
Google Alerts for Twitter. Using TweetBeep, you can create email alerts when a tweet matches your filters. You can get alerts when someone mentions a keyword (say, your brand or company name), when a tweet comes from a certain location. You can further filter tweets by positive or negative disposition, geographic location, or whether the tweet contains a question.
5. TwitPic
Makes it easy to post pictures to Twitter. You upload your picture to TwitPic -- via your phone, an iPhone app, or through a Web interface -- and TwitPic posts a link to the picture to your Twitter account.
6. Twitter Tools (WordPress plugin)
Twitter Tools is a WordPress plugin that lets you sync your WordPress blog to your Twitter account. You can pull in your tweets into your blog. Or go in the other direction, and automatically post a link to Twitter when you update your blog.
7. BackTweets
Find tweets that link to your URL (could be your domain or a specific webpage). Because of the 140 character limit, most links posted to Twitter have been shortened (and obfuscated). BackTweets unravels those shortened links to show you a list of tweets that link to your site. For example, here's the search for tweets that link to about.com.
8. Twitter Search
Twitter's search page lets you dig through the massive amount of tweets. You can search for tweets FROM or TO a specific user. Look to the sidebar to see keywords that are hot at the moment.