Metal Sonic's Quest by Jay Comix Inc

MSQ is a game where you play as Metal Sonic (it features the Sonic cast) as he, at first, tries to find all 7 Emeralds to restore his data banks. But, as you progress through the game, many zany characters will come in, making you do other stuff. There are quite a few secrets to find in the game, most are easy. There is also a second game mode, Multiplayer Mode, although it's easily beaten with only one person.

Legacy Player: The "Play online" link above uses the new Quest Viva WebPlayer, which is currently in Beta. In case of issues, you can try the old WebPlayer.