For the holidays, I gave myself an NSLU2, (AKA "Slug"), which a small, fanless Network Attached Storage device minus the storage, which attaches via USB. (The picture doesn't do it justice; the Slug is about the size of a moderately thick paperback book.) NSLU2s go for about $85 USD if you shop around.
The same folks who cracked the Linksys Router also figured out how to flash a new Linux into the NSLU2, yielding a tiny (albeit slow) Linux box that draws about 9 Watts and is absolutely silent. There's a thriving hacker community, which has come up with several hardward mods, including which resistor to clip to double the lock speed of the ARM chip from 133Mhz to a whopping 266Mhz.
I'm going to migrate several periodic tasks (e.g., network backups, periodic scraping of some web pages, etc.) onto mine, which will further plans for a silent home office by freeing up an older, larger, noisier box.