I home brew my own beer, and one important tool for brewing is a stir plate. It keeps yeast in suspension and aerates the growth medium for maximum cell count. To purchase one of these stir plates is easily $100 bucks and is probably overkill for my purposes. For around $10 and a few salvaged materials I can have a pretty snazzy stir plate that suits my purposes just fine.
You really don't need much in the way of materials. Once everything is gathered you can knock out the build in less than 30 minutes or so.
Assortment of washers, nuts, and bolts
cell phone charger
hard drive magnet
25 ohm rheostat (Radio Shack)
simple on/off switch (Radio Shack)
Knob (Radio Shack)
Some type of spacer material (washer, plastic, cardboard)
Tools you will need:
Drill and assortment of bits
wire strippers or utility knife