FreeNAS is close to releasing product-quality code for their BSD UNIX based Network Attached Storage software. First of all, I have to say I'm a big fan of BSD. While it is often confused with Linux, it is actually a branch of UNIX. Because of this, it has some security and reliability aspects that Linux distributions don't always have. The BSD distributions tend to be unkillable and very difficult to hack into. The main problem for new users has the ease of configuration and operation. FreeNAS removes this problem by providing a web interface and an automated installation process.
This software can function on a low-end computer -- it requires only 16 megs of RAM. You probably don't want to go with an extremely old computer simply because hard drive performance will be important. CPU and the amount of RAM are secondary considerations. But, if you are planning on using this storage device for archival or backup purposes, then even an older computer with a big drive will work wonders.
While the device may be single-minded in what it does, it has many ways to do its job. It can be access over the Internet and on a local area network by Linux, Windows and Mac computers.
If you are looking for low-cost storage, you cannot go far wrong with this system.