EasyTweets - A Twitter managemant app, features include: keyword search alerts, trends, manage multiple accounts, schedule posts, auto-post RSS feed content, Twitter alternate interface as a deck view, Google Analytics, auto-follow, bookmarklet to tweet page you are on, etc…
Socialtoo - email alert that tells me number of new followers and unfollowers, and my successful and unsuccessful tweets (what may have caused users to start or stop following), also see qwitter. You can auto-follow people who follow you, and auto-unfollow people who stop following you, blacklist users, Turn off automatic Direct Messages from other services, and more.
TwitProfiles - enter a Twitter user, and get a profile page with a follower button, here’s mine, similar to TwitterScore and other stat tools, like TwitterGrader.
For more Twitter buttons, check these out Get a TwitterButton, twitter buttons, and of course TwitterCounter.
Here’s a few I like:
Tweetzi - an alternate Twitter search interface
TwitterFish - a twitter auto-translation service. Login and any tweets in your stream that are of another language will be automatically translated on the fly. A very neat tool created by Andrew Gent.