We've seen a lot of Raspberry Pi projects that would benefit from a touchscreen, but it's generally not that cost effective to add one on your own. At $US35, Adafruit's tiny PiTFT screen is a reasonable solution.
The PiTFT Mini Kit is by no means the only cheap way to add a touchscreen to a Raspberry Pi, but it's probably the easiest. In the box, you get everything you need to connect the 320x240, 16-bit colour display to your Pi. You also get a handy guide to installing the software you'll need to actually use it. By no means is this an awesome display, but for the price it makes adding a tiny little touchscreen to your Pi super easy.
Adafruit PiTFT - 2.8" Touchscreen Display for Raspberry Pi ($US35) [Adafruit]