I am running a system based on an Atom 330 motherboard from Intel. It has 2GB of memory and a 320GB harddisk. I payed about 300 euros for the complete system, but you can probably get it cheaper. The motherboard with cpu was 70 euro.
I like it because it is powerful enough to do most of my daily computing. It runs an apache, a mailserver and serves as my desktop machine. I use a 1680x1050 Gnome desktop, fullscreen video, browser and email client. It has, in practise, completely replaced my normal (1300 euro) desktop. After I replaced the crappy fan that came with the motherboard it is now perfectly silent.
The whole system, under load, uses 28Watt.