Qnap is good as it comes with Slimserver pre-intalled (and some other bits which make it quite versatile). However, the other big consumer NAS player is Buffalo. You can install Slimserver on a Buffalo NAS as well:
I've just gone with Infrant. Bit pricier, but the X-RAID functionality means that I can have redundancy with a two-disk mirror that will grow just by adding additional disks to the unit. Also as available disk sizes grow I can hotswap larger disks in and the system will automatically resync to give me even more capacity.
It does say that it works with streaming servers as well, but check the blurb to make sure it does what you want. I've stuck it down in my cellar, working with a mixed Mac and Windows network. I need to work out how to get the Macs to connect automatically on startup though.
Cool looking box as well! Feels solid and well made.