I got an Intel SS4200. It’s a nice and powerful NAS, albeit silent.
I think Futjitsu-Siemens sells it with WHS, but Intel sells it with EMC custom’d Linux.
And then of course you can just get the bare thing and put your own OS on it, like I did 😉
It’s not exactly cheap, but the bang-to-buck ratio is decent.
I’ve used a Linksys NSLU2 before, which is awesome in it’s resourcefulness. But then it’s not nick-named “slug” without reason.