1. Twitter Timer – Need to remember something? Send timer a direct message, and it will tweet you back.
2. Twitter Mobile - A simplistic Twitter web interface orginally created for mobile phone users but also works well on desktop pc’s and laptops.
3. Tweet Volume – calculates and compares the volume of mentions in Twitter of words and phrases.
4. Twitter Counter – calculates and compares the followers of any Twitter accounts you select, for the last week, last month or last 3 months.
5. Twitterholic – cool tool that allows you to check the top twitters per city/region/country.
6. Tweet Deck – a desktop app that helps you organize your twitter account and makes communicating through twitter much easier.
7. Twhirl – a desktop app that has similar functionalities to Tweet Deck.
8. Destroy Twitter – another desktop app that has similar functionalities to Tweet Deck and Twhirl.
9. Twitpic – lets you share pictures on Twitter.
10. Mr. Tweet – finds relevant followers for you, great way of expanding your Twitter Follower Network.
11. Twellow – the Twitter yellow pages.
12. Twitbin – firefox extension that allows you to keep up with all of your Twitter conversations right from your browser sidebar.
13. Tweetr – allows PC & Mac users to upload and share files and photos.
14. Qwitter – useful app that e-mails you everytime someone stops following you on Twitter.
15. Twitpay – allows you to send and receive money through Twitter by making use of paypal.
16. WP to Twitter – a wordpress plugin that updates your status in Twitter everytime you post on your blog.
17. Twitter for WordPress – a wordpress plugin that displays your latest tweets on your blog.
18. Tweet Later – check out the free version, it has a really cool feature, it automatically unfollows someone that unfollows you.
19. SnapTweet – allows you to post your Flickr pictures to Twitter
20. LinkBunch – Need to share more than one link in a tweet? LinkBunch puts together all the links and provides you one link that you can share on Twitter. As you might have understood, this can be used not just with Twitter, but also with emails, IMs, SMS, etc.
21. Secret Tweet – allows you to Tweet anonymously.
22. TweetCube – allows you to post files on Twitter. The following are supported .zip, .rar, .pdf, .jpg, .png, .gif, .mp3 and .txt.
23. Twitteroo – another popular desktop client for Twitter.
24. Twittercal – allows you to post events from your Google (Google) calendar to your Twitter accounts.
25. Twessenger – updates Live Messenger status to reflect latest tweet.

Enjoy….:-)