Lately I've taken some old computer mice apart. There are a few interesting components which can be reused in there: On a ball-mouse, there are two pairs of infra-red emitters and collectors, and if there's a scroll wheel, there's a third set. On an optical mouse, there's the optical CCD sensor. In addition, there are several micro-buttons, and over at the Arudino Playground, they suggest even the controller chip and PS/2 plug might be useful.
Looking at the IR sensors, I thought I'd try to make it trigger on hifi remote. I based my first tests on the excellent IR tutorial over at ladyada.net. The problem was that the sensors from the mice are not standard three-legged VCC/ground/output. Rather, as Colin Fahey points out, the middle leg is VCC, while the two others are both output. The sensors in the mouse is in fact two detectors in the same package so the direction of movement can be determined. Makes perfect sense now, however I already melted to sensors before I found his article.
I also disassembled an optical mouse, and found the CCD sensor. Over at the EE Hompage, the have an extensive write-up on how it works. At Sprites mods, they have used the sensor as a crude scanner. Finally, Martijn The has successfully interfaced with an ADNS optical sensor using an Arduino. I guess I have another project waiting.