In Micro 4/3 (or Pentax Q/Nikon V and Sony NEX) you can use pretty much get adaptors in any lens mount...I've seen adaptors for Retina reflex/Voigtlander Bessamatic, Alpa, Zenit (m39), Kowa, Canon 0.95 rangefinder (it's a unique bayonet mount), Exakta bayonet, Rollei TM, ...not to mention adaptors for movie & video lenses (Arriflex PL, B4, Ikegami, C mount).
And there's the adaptors for medium format lenses - I know of ones for Hasselblad, Pentax, Mamiya 645, Pentacon 6/Kiev 60..........or adaptors for 35mm rangefinder lenses - M39, leica M and Contax G..there's even ones for the old Contax/Kiev mount Rangefinder lenses, and one for Nikon bayonet mount rangefinder lenses.
With Digital SLR's there are limitations - you can't use Minolta MC/MD lenses on a Sony/Minolta DSLR unless you use a adaptor with a lens in it, or you won't get infinity focus. Nikon and Pentax lenses work on most DSLR's, and as people have said - screw mount lenses work on many cameras..
It all depends on the flange depth of the original mount, and the flange depth of the DSLR mount....