BOBBY NEEL ADAMS (already posted on Mofi) combines pairs of photos of a parent and child, couples or the same person young and old, making one person from them. No Photoshop here - real physical cut and paste. (Caution: contains nipples). I like these three because they add a time dimension into a still photo.
PETER FUNCH took photos of the same places in Manhattan over 10 days or more and combined them invisibly, collecting together similar people, in an exhibition called Babel Tales. SEGEI LARENKOV combined photos from the siege of Leningrad, overlaying them onto modern shots taken 65 years later, from exactly the same locations. (Caution: contains corpses).