Linux and Samba still works.
Most versions of Linux support NTFS now so the Linux and Samba solution works.

Look at: NASLite, FreeNAS, SME Server, OpenFiler, Clarkconnect.
You could even use a full distribution like Fedora or Ubuntu (with Samaba) for more versatility.

You don't need much more than a 500MHz cpu to do this. [Inexpensive!]

Why does a 'build your own' need to be large?
Use a uATX board and a mini case.
In a pinch you could even make an old laptop work.

Best solution imo is a linux based nas/router eg NSLU2, WRT's, bubba, Thecus n****, linkstation pro, asus wl500gp etc etc, a lot have hacked firmwares or support debian based distros offering ext3 and/or full ntfs read/write.

With the added bonus of having a very cheap and energy efficient server/download/bittorrent box. Downside is you'll have to get your hands dirty getting it set up.