If you are old enough to have played coin-op video games at an arcade then the Internet Arcade is a pleasant stroll down memory lane:
The Internet Arcade is a web-based library of arcade (coin-operated) video games from the 1970s through to the 1990s, emulated in JSMAME, part of the JSMESS software package. Containing hundreds of games ranging through many different genres and styles, the Arcade provides research, comparison, and entertainment in the realm of the Video Game Arcade.
The ability to play some of these games in the browser is really impressive.