git annex (Score 1)

git annex is an open source project that lets you distribute files around various media (including external HDs, Amazon S3, SSH-connected computers, etc.). It has an fsck command for checking that your data still matches its checksums.

There's a GUI interface that makes it a lot like Dropbox, where you just add files to a folder, and they are sync'd.

It works on OS X and Linux, with an alpha for Windows.