FastLock is a newly released jailbreak tweak, which allows the user to quickly access certain parts of their phone by simply double tapping the home button twice. You gain the ability to compose a tweet, text, phone someone, send an email and some others. For the price of $0.99, the premise of this tweak is quite nice. Of course, it has been done before, this is just another variation on the same idea. However, I say nothing wrong with that!
You can pick up FastLock in the ModMyi Repo.