Sonic World is a fan-game by a team headed by Ozcrash (the same guy who brought you BlitzSonic Heroes). When I downloaded the game, I was just looking for a nice BlitzSonic experience, but when I actually played it for the first time, I was blown away at how good it was. The game was incredible. There were 18 characters to choose from, each with their own abilities (though I was a tad disappointed that every one of them used the Spin Dash rather than having some as challenge characters like Amy in Sonic Advance), and 5 great stages, each with 5 missions to complete. It was so fun checking out how each character played through each stage. There are even additional downloadable stages for even more variety!
Unlike other BlitzSonic games, the Badniks can actually get quite hard to handle, and each have distinct behaviors. The problem with the game closing out when you just try to get back to the main menu is absent. And the stages are far livelier than in other BlitzSonic games. This is likely the best BlitzSonic engine EVER.