Keyboard replacement wherein you trace words on the keyboard instead of typing out the letters via tapping. For some it's a lot faster, for others it's more of a gimmick or a novelty. I'm using it more since my beloved ShapeWriter (a similar app) stopped updating. Try it out, you might like it. If you do, though, be sure to give it at least a day before deciding if you like it or not, since there is a bit of a learning curve.
Written on an HTC Incredible using Swype.