Twitter is everywhere nowadays, I love it. The amount of useful resources, tools and apps available are incredible. I think i have listed the best and most useful, let me know if I have missed any.
Description : FutureTweets.com is a free service that lets you schedule your Twitter messages. Send it at a specific time in the future or send a reoccuring Tweet daily, weekly, monthly or yearly!
Description : Qwitter allows you to find out who has quit following you, and at what message they stopped. This service is definetly for twitter-addicts (I checked).
Description : Just Tweet is a directory for Twitter users, it makes it easier to follow people with similar interests to your own.
Description : Typetweets is an up to the minute look at the typographic discussions happening across Twitter, and a great way to discover interesting type-themed bits on the web.
Description : This site gives real time Twitter User stats and analytics. Really interesting viewing.
Description : With this service you can create an event and allow your users to RSVP back to you.
Description : A simple but usefulidea from Twitly, basically you can seperate everyone you follow on Twitter into groups.
Description : Your Twitter Ratio is the ratio of your followers to friends (or people who you follow). It is measured with the TFF Ratio (Twitter Follower-Friend Ratio). The higher the ratio, the more Twitter heat you pack.
Description : Tweetlists shows you the most popular tweets in the last 24 hours.
Description : SecretTweet was created to allow Twitter users to share secrets anonymously.
Description : @answerme is a little tool to help you track questions you ask on Twitter. Imagine Yahoo! Answers lite built on Twitter and you've got a good idea of what it is doing.
Description : Add a daily updating TwitterCounter to your blog so everybody can see how popular you are. First Check your Twitter Account by entering your username in the search box, then get the button code, and copy and paste the code on to your blog.
Description : TwitPic lets you share photos on Twitter, you can post pictures to TwitPic from your phone, or through the site itself.
Description : View a live via Google Earth of live tweets as they happen.
Description : Enter up to 5 keywords into the boxes and see how often they appear on Twitter. I am not really sure what you could use this tool for (other than marketing), but it did keep me amused for a good 15 minutes. Try it.
Description : Foamee tracks who you owe beer or coffee to (and vice versa) via Twitter.
Description : The simplest of simple. It asks the question: Is twitter down for just me or everyone? I have bookmarked it, you never know.
Description : Tweetake allows you to back-up your followers, people you are following and Tweets with just one click.
Description : Advertisers create campaigns providing a message and some keywords, it finds matching twitterers are selected, costs are calculated based on # of followers and hotness of the topic.
Description : Twiggit is an automated service that lets your friends on twitter know what articles you digg.
Description : You Use LazyTweet via Twitter and leave a question or request of some sort and wait for some feedback.
Description : Twitag is a tag finder for twitter, the goal is to facilitate and organize the most recent content tagged by users.
Description : You can track your biorythm easily via twitter.com. You can watch a line chart of your day-to-day condition and messages at http://highxlow.com/twittername
Description : If a link is posted within a tweet,it is collected and numbered. For tweets with tinyurls we also reverse the URL so that you can see in your browser where the tweet realy is linking.
Description : EmailTwitter.com allows you to post Twitter updates and retrieve your Twitter timeline via your cellular phone or other e-mail enabled mobile device, without incurring SMS fees.
Description : When you give your Twitter credentials you are supplied with a TwitterMail email address. For instance firstname.lastname@example.org. If you send an email to that address it will be posted to Twitter.com.