I once bought a 2.5" USB HDD in order to use the HDD itself in a laptop. The case ended up being used for a homemade USB battery pack/LED flashlight/green laser pointer that I still use today.
I bought a $1 DSL modem at a garage sale many years ago. I haven't reused the case yet, but I have already reused some of the internal parts and the wall wart.
How about a cheap set of Harbor Freight automotive test lights in order to take out the bulbs and put in my own electronics? I modified one with a bicolor LED along with a constant current circuit that will work on 5-600V, making it useful for lots of things relating to power supplies from checking the polarity of a wall wart to discharging mains voltage capacitors.