6502 XOR Disassembly, by Amf
A commented disassembly of the Beeb version of XOR, by Amf.
XOR solutions and commentary on Youtube
An extremely enlightening series of videos playing through each level, by SentinelProxima.
Wikipedia has a page about XOR.
XOR in Java, by Rob Veldhuyzen van Zanten (website has both English and Dutch text)
The first map ("Together") appears to be one of Rob's own devising!
Chroma, by Amf
Chroma is an abstract puzzle game that bears some similarities to XOR. It can also be configured to play XOR levels. An additional download gives all the XOR levels and all the graphics sets.
It is open source, and available for Windows and Linux.
Enigma, by Simon Tatham
A puzzle game that contains aspects of Boulderdash, Sokoban and XOR.
The Stairway To Hell
BIG collection of BBC games and gaming resources.
The BBC Lives!
Another excellent Beeb site.