Combining git-annex with Shamir secret sharing is an useful way to securely back up highly sensitive files, such as a gpg key or bitcoin wallet.
Shamir secret sharing creates N shares of a file, of which any M can be used to reconstitute the original file. Anyone who has less than M shares cannot tell anything about the original file, other than its size.
Where git-annex comes in is as a way to manage these shares. They can be added to the annex, and then git-annex used to move one share to each clone of the repository. Since git-annex keeps track of where each file is stored, this can aid later finding the shares again when they're needed, as well as making ongoing management of the shares easier.
Note that this conveniece comes at a price: Any attacker who gets a copy of the git repository can use it to figure out where the shares are located. While this is not a crippling flaw, and can be worked around, it needs to be considered when implementing this technique.
Here is an example of this method being used for a ~/.gnupg directory: http://git.kitenet.net/?p=gpg.git;a=blob;f=README.sss