I've built enough hobby projects with Arduino that I've noticed that they aren't that cheap. Still, I rarely scavenge old Arduinos out of finished projects but I have given several not yet used ones away.
Sensor Server in many incarnations starting with a single thermometer hung off of the usb port of an old linux server.
Mini Ray animated a foam core cutout of Ray King with joints at waist and neck and two white light LEDs for eyes.
Spinometer watched black tape on my bicycle rollers go around so as to measure my cycling performance.
Dome Cam spun an old iSight camera around on two axis without tangling up the cords. Quartz Composer righted the image for me.
Wikiduino Server resurrected my abandoned first Sensor Server implementation to serve soil temperatures to wiki.