make it a file server, or server for anything. 500mhz P3 is not too quick but. . .
you can set it up to serve all your media files. My old athlon xp is now a server, she has all the mp3's, movies, pictures, documents, etc on her, and she has the larger drives, while my other computers only have 60 or 80gb drives. i also run the printer off of her, and the slimp3 server. kinda takes the load off my main computer for more important things (like gaming
it also allows central storage of everything since i have several different computers. that way, i have access to everything.
if it was a little quicker, you could run a game server, streaming media, etc, etc. . .