Here's a few that I can recommend:
TweetReach - This app helps you work out how far your Tweets are reaching - how many people are seeing & interacting with them. All you need to do is insert a URL and it will do the rest!
Twenty Feet - Great for tracking how many mentions you're getting, as well as many more stats such as retweets, replies and follower growth.
Twitter Counter - This is a handy tool for tracking your follower growth, although be careful - followers doesn't always mean engagement or clicks!
Social Bro - A tool for analysing the followers of your followers. Next level stuff.
Twitalyzer - This is a really indepth Twitter analyser, and perfect for business accounts.