As well as the Sonic and Mario games in my son Peter's FlashyOnlineGames.com site, there are also lots of retro computer and arcade games rendered in Flash so that you can play them online in your web browser.
Here are some classic 1980's computer games from the Commodore C64, Commodore Amiga and Sinclair ZX Spectrum:
Great Giana Sisters
And here are some 1980's arcade games:
(See also here)