I think that's why so many people go with the older HP microservers. Super cheap, nice form factor, just add ECC RAM and HDDs.
It's okay as a NAS that mostly sits idle and occasionally serves up unencrypted data at GigE speeds or less. For more demanding tasks, it's woefully underpowered.
Still, a box is a box :P
Of course it always depends on the use-case, but for most people at home it's sufficient. I use it as a Minecraft and media server.
The closest to a classic HP Microserver is the SuperServer 5028A-TN4, although that one uses the Atom chips that have been recalled.
There's also the much more powerful SuperServer 5028D-TN4T.
They also have a bunch of older models whose names escape me, but can be found by searching for "supermicro mini tower".