Great for anyone that grew up/played on the original NES/SNES etc.. They have a ton of games on the various sites. It's all free, java based and saves are available. Some work better than others.
Some games:
Tecmo Super BOWL!
1) Super Mario Bro's
2) Super Mario Bro's 2
3) Metroid
4) Double Dragon
5) Legend of Zelda
6) Super Mario Bro's 3
7) Tetris
8) Mega Man 1-5
http://www.nintendo8.com/
SNES
1) Donkey Kong Country 1-3
2) Super Mario World
3) Legend of Zelda a link to the past
4) Street Fighter II + Super Street Fighter II
5) Chrono Trigger
http://snessy.com/
Here are the other consoles
Sega + gamegear
http://mastersystem8.com/
Gameboy
http://gbemul.com/
Dos
http://dosdose.com/
Commodore 64
http://c64i.com/
Enjoy