FreeNAS is a small, powerful, full-featured implementation of FreeBSD as a network-attached storage device. (It also happens to be January's Project of the Month at SourceForge.net.) If you're a Linux user, the BSD-speak used for devices and such might give you pause, but other than that small caveat, installation and usage shouldn't be a problem. It's powerful enough to be used in the enterprise, but it's friendly enough so that even a typical home office user can take advantage of it.
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