I own a VIA EPIA-LN 10000EG, placed inside a Morex Cubid 2677 V3.0. With 1GB RAM this one costs around 230€ - harddisk not included. It runs 24/7 here since ~1y (using FBSD) and is almost noiseless due to passive cooling and a 2,5" disk. I've chosen this little one since it has a TV-out, although I've not used it yet
However if I'd had the chance of getting a new one, this would definitely be a VIA PICO PX 10000G inside of a DL-431DV Pico-ITX case, which will sum up to ~350€ (1GB RAM,w/o HD).