As I’ve mentioned in a previous post, I grew up on the original NES. I found a site the other day which claims to have “every Nintendo, Sega, Gameboy game and more!”. On this site you can play these games through a Flash emulator. The site is http://www.everyvideogame.com/ . I haven’t spent much time on the site but it worked fine for me. My only complaint is that the games are in a small window but it might be possible to play them fullscreen.
Another site I found amusing that relates to the NES is Speed Demos Archive. The site is dedicated to not just the NES but all different systems in which players find and compete the quickest way to beat a game. This is somewhat misleading as often these runs are done with a player finding glitches in the game or piecing together the best times they get for each level and using them as a speed run. For more info on speed runs, here’s a faq.
Here is an example of a speed run in Super Mario Brothers 3. It is completed in just over 11 minutes.