This game has been around for a long time. It can be enjoyed online and offline. Now before you go, "LOL SANIC?!" this is not a new gen sonic game I tell you. The graphics pass and the music is nice in my opinion. You can make gifs with the press of F9 and take screenshots with F8. You can visit the message boards and check out the modifications (WADs) people have made to enhance gameplay. If you're not intimidated by it like me, you may learn to make your own WADs. Here are some random gifs I picked out from when I was playing it. May include modifications I added.
In this gif, I added the HyperMysticSonic wad (It's a joke character wad poking fun at how super saiyan sonic goes) and the tails doll wad.
Ah. One of my favorite wads if not my most favorite. The portal wad.
Hope I convinced at least one person. I love to play this game when the internet is down or when I'm simply in the mood for some old gen sonic. (You can be tails or knuckles too btw.)
http://www.srb2.org/