Also Darcs, which seems to have some possible advantages (simplicity, its "theory of patches" based merges, MS Windows support) over Arch. My gosh, first there was Bitkeeper, now also Arch, Darcs, Monotone, and also SVK and something called Codeville, which I just stumbled across. All of these are attempting to do distributed version control, but the, ah, wealth of different tools here makes picking one a bit complicated.