These were posted by Rhiannon / Kammer here a while ago but were new to me at least:

http://www.stonehengelaserscan.org/carvings.html
http://www.britarch.ac.uk/ba/ba73/feat1.shtml

- and interesting because they don't seem to have been considered in the recent Parker Pearson or Wainwright theories - or in the bluestone debate.

A few years ago, I remember we read a reference to another carving - on top of one of the lintol stones - (in the previous incarnation of the pre-history gallery at the British Museum) but this doesn't seem to be mentioned anywhere else.