Awesome, what speed? Put loads of RAM in it (if you can get the bits cheap and if it's not already maxed out), and make sure it's running at least tiger, Leopard works good too though, and is a bit more supported.
Then, use it for whatever you want. Mine mainly gets used for streaming movies to the xbox, listening to music, and internet stuff now, but it has, at various times been used to edit video, record, mess with music, make DVD's, edit photos.
Possibly more inspiration on lowendmac.com.
Or, as mentioned, pass it on to someone who needs it.