DOS Flavours

DR-DOS Caldera DOS (Previously Digial-Reseach, then Novell DOS)
FreeDOS Freeware DOS Project (in beta)
MS-DOS Microsoft's DOS
PC-DOS IBM's DOS
PTS DOS 2000 Paragon Software's DOS
ROM-DOS Datalight's DOS PC-DOS clone for ROMs
RxDOS DOS by Mike Podanoffsky