Inklet turns your MacBook trackpad into a tablet, with handwriting recognition
Inklet can do a lot of the things a full-sized tablet can. You can use it to draw in Photoshop, and even take advantage of OS X's built-in handwriting recognition. Although a finger works just fine, the Pogo Touch stylus (sold separately, of course) is pressure sensitive, allowing you to move the cursor without drawing or apply variable-width strokes. To use Inklet, you'll need a multitouch trackpad - metal unibody MacBooks and the most recent generation of white MacBooks have them - and OS 10.6 Leopard.