I was pulling some data from PCAPs recently which had simple single-byte XOR's applied. It was kind of a pain to keep moving the data from one tool to another to brute force it so I went ahead and wrote and submitted a script to Sweetscape's 010 Editor which brute forces the XOR key based on a string you supply.
They accepted the script and published it on their website so it should now be available within the 010 Editor if you want to download it.
Their script language is quasi-C, which is how I originally wrote it, before finding all of their sweet API's. It seems pretty powerful and makes me <3 010 that much more.