Only yesterday, a third product that I worked on over the past little while called Windows Storage Server 2008 R2 Essentials was announced, on hardware provided by Western Digital.
Windows Storage Server 2008 R2 Essentials is a mid-way point between Windows Home Server, and Windows Small Business Server 2011 Essentials. And by in between, I mean you have a business class Home Server. So you have media streaming, but no domain controller, and a limit of 25 users instead of 10 that Home Server has.
One other unique feature it has is the ability to join a domain, that Windows Home Server does not have. You can join any size domain, but limit (via AD security group) the number of users down to 25 who can use the NAS box. You can only use it for up to 25 users.
This is a great solution if you want to add client backup to 25 clients on any SBS 2008 or SBS 2011 Standard networks, or just a standard server network. Furthermore, it does have RWA with it, which means you can add an RWA solution to your standard server network if you want.
The WD Sentinel DX4000 looks pretty awesome from a hardware perspective as well. You can jump on over to the Western Digital WD Sentinel DX4000 website to learn more.