There's loads of info here:
http://forums.hexus.net/shopping-ret...ver-100-a.html
The general consensus seemed to be Openfiler, and that's what I've gone with. It's very good, extremely stable (and beyond that I'm not sure I'm particularly fussy), but I have to say it wasn't the easiest to set up.
Had to muck around with quite a lot of command line stuff to get it to actually work properly, and getting it to work on a Windows network takes far more configuration than you would think was necessary. Not to mention that documentation is paid for, and therefore if you're a cheapskate, lacking.