Museum of Online Retro Games here you can even play DOOM! It has all the goods!
Emulator Frontends Some of these are pretty damn cool.
The Best Emulators Out There The NES Classic is sold out, but these emulators do the same thing for free
The Emulator Zone A collection of all of the emulators available for each console
Zophar.Net My favorite place for emulators and hacking tools
Romhacking.Net My other favorite place for rom hacks (patches) and hack tools.
See other links in my "New and Indie" section for roms.
The ISO Zone The world's finest retro gaming resource
ArcadePunks.Com Lots of collected packs of game roms for each console
CoolRom.Com boasts being the largest resource on the net. also has emulators
EmuParadise.Me Several roms, isos, and games for all consoles and arcade M.A.M.E. systems
Play Super Mario Bros/Flash version in your browser.
Super Mario Bros X a remake of the Super Mario Bros/Super Mario World games where you make your own levels!