Twitter Mashups are arriving at the plenty. With some you struggle to see their worth, but recently I have discovered some that I can immediately see some clear uses for:
Dwigger threads Twitters and enables users to vote on tweets. I really like the fact you can also filter down for a selection of areas (the UK being one, New York another).
Twitter Grader may at first just seem to be satisfying the ego of Twitterers, however with no stats available within Twitter it could help measure the impact of a campaign that incorporates Twitter, as could Tweetburner which enables you to measure the number of clicks on a URL you post within a tweet.
Other of my favourite Twideas:
TwitterFeed automatically posts a Twitter when you add a post to your blog or other RSS feed.
Twellow a directory of Twitters by subject.
Twitt Earth shows Tweets as and where they are coming from, not really useful but one of my favourites and makes a great screensaver.
On my iPhone I use Twinkle which lets you filter twitters from 1 to 252,000 miles from you.
This post is clearly not an extensive list of Twitter apps, there are plenty of sites that do that. I wanted to share my favourites and my excitement over the amazing Twideas that Twitter inspires and will continue to as the microblogging phenonemum continues to grow.