I own an nslu2 which fits the bill, except I'm not very happy with its performance. Lots of other people seem to be, so give it a go if you don't need a high performance NAS. It was one of the very first to come with a linux firmware, so there's a thriving linux community around it, and people have modified it to run varied things.
There are two main replacement
firmware images available for the
device: the first is Unslung which is
based on the official Linksys firmware
with some improvements and features
added. Optware packages are
available to expand functionality. The
other is SlugOS/BE (formerly
OpenSlug), which is based on the
OpenEmbedded framework. SlugOS/BE
allows users to re-flash the device
with a minimal Linux system including
an SSH server to allow remote access.
Once installed, the operating system
must be moved to an attached hard disk
due to the lack of space available on
the Flash memory. Once this has been
done, a wide range of additional
packages are available to be installed
from an Internet repository.
It is also possible to run OpenWrt,
Debian, Gentoo, FreeBSD,
NetBSD, and Ubuntu on the
The ability to run an unrestricted
operating system on the device opens
up a whole new range of uses. Some
common uses are a web server, mail
server, DAAP server (iTunes), UPnP AV
MediaServers, BitTorrent client,
FreeSWITCH, asterisk PBX and
network router (with the attachment of
a USB network interface/USB modem).
I used to have it running with an Unslung firmware booting from a USB stick, but recently converted to running Debian off the attached USB hard drive.
The nslu2 makes no noise and draws minimal power.