Autopostr – This service lets your friends in Twitter know when you post a new picture on Flickr.
EmailTwitter – With this service, you can send tweets to Twitter from your cellphone without incurring SMS charges.
GroupTweet – Allows you to post private message to a group of Twitter friends.
Hashtags – You can add tags to your Twitter posts with this service.
HelloTxt – This service enables you to update Twitter, a host of other micro blogging sites, and social networking sites with one click.
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.
LiveTwitting – Helps you cover a conference live on Twitter. Similar to liveblogging, only cooler.
MicroRevie – Post reviews on twitter accs and this service will turn them into microformats.
Mobypicture – A service to post to your blog and micro-blogging sites like Twitter through your mobile phone.
OutTwit – A tool for Outlook that will send the latest tweets from your friends as incoming email messages.
SecretTweet – Post to Twitter anonymously. If you really need to.
SnapTweet – Allows you to post your Flickr pictures to Twitter.
Snitter – an Adobe AIR based desktop client for Twitter compatible with Mac and Windows. The Linux version is in testing.
Twhirl – Twhirl is a popular desktop client based on Adobe AIR. It has a lot of impressive features including multiple Twitter accounts, connect to Friendfeed, post image to Twitpic, post to Pownce and Jaiku simultaneously, and search using Tweetscan.
TweetCube – This service allows you to post files on Twitter. Files that can be shared include .zip, .rar, .pdf, .jpg, .png, .gif, .mp3 and .txt.
Tweetr– an AIR based Twitter client for MAC and PC. You can use this to send files upto 100MB.
Twibler – posts your ebay listings to Twitter automatically.
Twideeo – You can post videos to Twitter using this service that lets you upload the vid to their site and then generates a link to your video.
Twit+ – You can tweet pictures, videos, and files using this service publicly or privately among your friends on Twitter.
Twitxr – Another neat way to post a picture on Twitter, Facebook, or Picassa.
TweetChannel – You can add TweetChannel as your friend in Twitter to help you channel your various posts.
TweetLater – An useful service to schedule your Twitter posts for the future.
Twitpic – Share pictures on Twitter using Twitpic.
Twitsay – You can tweet an audio recording to Twitter using this new and popular service.
Twits Like Me – You can use this service to find people on Twitter who share your interests.
TwitterCal – This service allows you to post events from your Google calendar to your Twitter accounts.
Twitter Feed – This tool allows you to send your blog atom or RSS feed to Twitter.
TwitterGram – Post an mp3 on Twitter using this service.
Twitteroo – This is another popular desktop client for Twitter.
TwitterIM – Using this tool, you can tweet from Windows Live Messenger and ICQ.
Twitterlights – This tool allows you to highlight snippets of text from any webpage and send it to twitter. The url of the page also gets converted into a tinyurl and gets included in your tweet.
Twitter Reply – With this tool, you can send your Twitter updates on a secret email address, or through Windows Live Messenger or ICQ.
Twittershare – This desktop/web application also allows you to post pictures on Twitter. Works on Mac as well.
VisualTwitter – Allow you to post pictures on Twitter.
TweetWire – TweetWire grabs the freshest links posted on Twitter and displays them Yahoo News page style.
Twitter100 – Just like in your personal start pages, this tool will allocate a box to each of the people you follow on a single page and display their latest tweets.
Twitterator – Twitterator enables you to follow a group of people at one go.
Twitter Digest – This tool allows you to subscribe to a message stream from a Twitter user of a group of users on the web or through an atom feed.
Alpha Twitter – This site provides you with the top links on Twitter.