Here's one really cool way to recycle an old Mac.
Etsy seller pixelthis has turned the panels of retired Macs into clocks. pixelthis has already sold one clock made from the side of a G3. The seller currently has a clock made from the side panel of a G4 for $59USD and from the front cover of a G4 for $57USD. There are other unique clocks including those made from hard drives, circuitboards, bicycle wheels and old records.
[Via Macgasm thanks to Josh for the tip!]