This is the not very exhaustive list of where you can play old consoles online. If you have been living under a rock, then you may not know about websites using emulation to allow visitors to play old console games on their sites. Some of these sites are, of course, highly illegal and you will probably burn in hell for taking part in such travesty.
Now that that's out of the way, here is the list.
1. Vizzed - This is the most awesomest online emulation site on the web today. Go take advantage of them now, while Nintendo is not looking. Vizzed has almost all retro console--from the NES and the SMS to the Gameboy advance and the N64. Go visit them now!
2. Play Sega - This is a legal endeavor by Sega Corporation. They have a few titles you can play for free, but they do have a VIP-like system where you buy points to play premium games. It's still worth your time to check out if you are a Sega fan.
3. Game Oldies - This is another highly illegal site, I'm certain, that has some Sega, NEC Turbographics16, and SMS games.
4. Virtual NES, PlayNES, and Nintendo8 - Guess what these sites emulate? I'm sure Nintendo of Japan have no English speaking employees and Nintendo of America is not connected to the Internet at all.
Well, that's all for now. Leave a comment if you have any other site you want to contribute to the list.