During the course of your English-speaking life, you might have noticed that certain letters don't go after others very often. For example, it'd be rare to see a word that features the letters J and G right after each other.
So when you're typing, it's a waste of space to have every letter the same size. ThickButtons intelligently predicts what letter will go next, and makes that bigger - making the letters that just don't go after them smaller. If you hit the wrong one, and go back, it'll make the letters either side of the one you hit bigger, too.
It's a free download on the Android Market (search for ThickButtons), and there's a version for Windows Mobile in the works too. The company is talking to RIM for a BlackBerry version, and there have been "early discussions" to license the technology to LG and Samsung. Unless Apple changes its policies, there won't ever be a version for the iPhone.
In our brief testing, we found the app to be more useful for slow typers than fast ones - the pause to redraw the keyboard makes the experience lag just a little - but that little is enough to be a pain if you're writing a long word and know where the letters are already. We'd love to hear your thoughts on the app in the comments, too.