Now here's a fascinating app -- Inklet is a new Mac app by Ten One Design that purports to turn your Macbook's trackpad into a tablet-style input. The program uses the multitouch trackpad interface to sense where and when you're touching the pad, and then translates that into "drawing" marks just like a Wacom pen tablet. Unfortunately, the trackpad doesn't have sensitivity built-in (it can only tell when you're touching, not how hard you're pushing), but with Ten One's Pogo Sketch stylus, which I presume sends pressure information back to your Macbook somehow, you can get that "line thickness" functionality -- press harder for a thicker line, or less for a thinner line. Pretty awesome. It's not as big or probably as sensitive as a commercial Wacom tablet, but just for doodling and messing around, it's a much cheaper solution.
Inklet requires Snow Leopard to work, so if you're running anything pre-10.6, you're out of luck. The software is $24.95, and their website has been brought to its knees by recent press, so you can download it from Apple's site if you can't reach it the official way. The Pogo Sketch stylus is only required for pressure sensitivity, but it's a very reasonable $15. Like I said, if you're like me and want a Wacom but haven't had the cash or inclination to spend on it, Inklet could be the stepping stone you're looking for.
We'll have a full TUAW review of Inkjet within the next few days.
Apple MacBook Pro 15-inch with Retina Display (mid 2014)