I have a small selection of emulators for download here. They should cover your basic classic gaming needs. They are also what I consider the best for their system. If you need them for other systems, or for some reason, these don't work on your system, then you should try Zophar's Domain
Sega Master System
For Microsoft Windows 2000, XP, and Vista
MekaW version 0.72
For Microsoft Windows 9x, NT (or older 2k, XP computers)
Chasms version 0.30
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Sega Genesis/Mega Drive
For Microsoft Windows 9x, 2000, XP, and Vista
Gens version 2.14
GensPlus
KEGA version 3.51
For Ubuntu 6.06 (Dapper Drake) thru 8.04 (Hardy Heron)
Gens version 2.14
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Sega Saturn
For Microsoft Windows 2000, XP, and Vista
GiriGiriByAlex (the only Saturn emulator that I've gotten to work).
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Nintendo Entertainment System
For Microsoft Windows 2000, XP, and Vista
Nestopia version 1.39
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Super Nintendo Entertainment System
For Microsoft Windows 9x, XP, and Vista
ZSNES version 1.51
For Microsoft Windows 2000, (also works with 9x, XP, Vista)
SNES9x