Web server, media server, game server. Take your pick, or choose all three! I did the same thing with my spare single core 3.4ghz. That's where I host my avatars, files and media. I also use it for bit torrent downloads to take the strain off my gaming machine, which also let's me keep the gaming machine shutdown at night when I am not using it essentially prolonging it's lifespan. And if yer worried about another monitor/keyboard/mouse cluttering a desk: just hook them up to install OS and other proggies, but put a VNC server up on it and voila, a headless server!