eMacs are still great for kids--very good for Mac OS 9 CD-ROM edu games and basic web sites. Not very good for Flash-based sites like Club Penguin though. Maybe you know someone with a 5-9 year old who would get some good use out of it? If it has a DVD drive then it even doubles as a TV/DVD combo.
Other ideas: iTunes jukebox, dedicated bittorrent download machine, run folding@home or SETI.