There are a great many Super Mario Brothers and Sonic the Hedgehog Flash online games on the web, most of which are pretty good fun to play, and a few of which are so good they actually rival the real console games that they are inspired by.
You can play over 150 of the best ones at my MarioSonicGames.com website for free.
Here are what are universally considered to be the "All-time Top 5" Mario and Sonic online games:
New Super Mario Flash
Super Mario Sunshine 64 Flash
Ultimate Flash Sonic
Shadow the Hedgehog Flash
Super Smash Bros. Brawl Flash
Enjoy!