1 site that doesn't allow mass amounts of pop-ups to occur or send you in a loop of crap sites is
www.qualityroms.com . Not only do they have a nice selection of SNES, Sega Genesis and NES games but the downloads are up all of the time and there's no slowdown.
the-underdogs.org has some odds and ends of roms under their "Non-PC" section. Hope that helps