ZSNES Sourceforge Page
ZSNES is easily the most popular SNES emulator there is. It has its own GUI, VERY good compatibility, boatloads of features, and all that good stuff. The only disadvantage it ever had was an utter lack of portability. It stayed under DOS for ages, then recently they began a port to Microsoft Windows. Now that they've released the source under the GNU GPL, however, there is a Linux port in the works and a Sourceforge project for it. You can get it through CVS, HTTP, and many other ways. It's a great emulator, and I was overjoyed to see the source released. Beaujolais to the ZSNES team for making the right choice!
SNES9x is currently the most ported SNES emulator around. There are
versions of this emu for Amiga, Macintosh, SunOS, BSD, Linux, BeOS, and even
legacy DOS and Windows systems, to say nothing of the N64 port for those lucky
enough to own a dumper and a flashROM.
The main reason that there are so many ports of SNES9x is that its
source code is freely available. While it's not updated all that
frequently, there is at least some code out there for porters to use. Be
careful, though, since the SNES9x License forbids commercial use.
M.A.M.E. (Multi Arcade
M.A.M.E. is an incredibly successful emulator meant to document the
hardware and software of arcade games, since the ROM boards eventually decay
to the point of uselessness. The moderators consider it a happy little
bonus that one is actually able to play the games, as well.
The M.A.M.E. license allows use and modification of the source, but
puts a lot of restrictions on distribution. Read the license, of course,
before doing anything with it, and make sure you send any improvements to
the moderators (that's how M.A.M.E. got where it is, after all).
Source code info (and vague
This is a relatively new NES emulator that very dearly wants to be
like SNES9x. It is NOT done by the same crew, and does not have any sort of
clear-cut license (although, like SNES9x, it forbids commercial use).
Source is released on an "as we feel like it" basis, again, much like
SNES9x, and the operation seems to be a real hack job thrown together by
SNES9x followers. It is, however, a decent emulator with available source
code and a bright future ahead of it (it will probably be ported like
nobody's business if it catches on).