I'm a bit late to this party, but the NSLU2—affectionately known as the 'slug'—is a piece of kit you can't afford to be without on your home or small office network.
Not much bigger than the palm of my hand, and cheaper than a ticket for a Test at Lord's, the NSLU2 is a small fileserver that serves files from attached USB disks.
What makes it particularly special is the large amount of alternative firmwares built by Linux open source developers, which allow you to extend the functionality of the NSLU2 beyond merely serving out files via SMB (the Windows file serving protocol.)
So what's the big deal for me? Well, like most laptop-slinging folks whose home network is predominantly wireless, I want my backup disks to sit on the network, available when needed.
It turns out it's very difficult to find a standalone network disk that properly supports Unix file system semantics such as symbolic links. Most just support SMB file sharing, with attendant limits such as no links and no files over 2GB. (I found this out the hard way with Apple's Airport Extreme.)
With the NSLU2 I was able to install the "Unslung" alternative firmware, and install good old NFS, making my backup disks available in a normal way to Linux and OS X machines alike. (How we used to complain about NFS back in the early 90s, but Windows file sharing still makes it look good!)
In the great tradition of open source there are multiple choices of Linux distributions you can install. As it was my first time round, I went for Unslung, which preserves as much as possible of the official Linksys interface, but lets you extend it. Next time, with a better idea of what I'd use the box for, I'd probably plump for Debian.
Inside the case, the NSLU2 is in fact a tiny Linux machine with 32MB of RAM and an Intel XScale CPU. This turns out to be plenty enough resources to serve files on a small network. Aside from my prosaic needs, the NSLU2 has been put to several more innovative uses, such as a music server for Apple ITunes and a 4-line home telephone exchange.
I've been astounded at the applications people have devised for this little box. Being fairly cheap makes it a great candidate for home automation projects. It's a great example of how limiting resources fosters innovation. Remember how games on 8-bit microcomputers were so much more ingenious than those on their more well-resourced successors?
So, I may be a little slow in finding this little hardware gem, but I wholeheartedly recommend it.