Here are some emulators I've ported from DOS or Win32 to Linux.
All of them need the Allegro library which you can get here , with the exception of Dega/SDL which needs SDL and FCEU which needs Opie/Qtopia. Allegro, in turn, needs esound and the Audio file library which should come with your distribution.
If you plan to compile the stuff yourself (don't try if you have a weak stomach), you will need a few more things. Check out the Allegro homepage for details.
All of the emulators come with full source, and RAINE, c2emu,
Dega/SDL and tgemu make more or less heavy use of IA32 assembly, so
don't expect to get them running on non-Intel CPUs easily.
Sources removed on request by Charles MacDonald