A Sonic-themed game created by Sonic Team Junior. It was created to be a fangame, and a sequel to it's lesser known first game - SRB.
SRB2 Stands for : Sonic Robo Blast 2. As you can guess, SRB Stands for Sonic Robo Blast.
The fangame, in single-player, consists of 18 zones (including acts, eg: GFZ Act 2), 4 secret levels, 8 Special Stages, and about 28 zones that are a faithful remake of the levels of SRB.
There are six characters (including some unlockables) if you exclude modding: Sonic, Tails, and Knuckles (7 if you include Sonic & Tails).
There is also a Multiplayer Mode, that can be used as either Co-op or Online Servers.
As semi-mentioned previously, This game has been modded to add more characters, stages, and more. So, as expected, most servers are modded. Of course, there are a small few that aren't, but it's usually quite rare.
Online players are reasonably friendly, and from personal experience, hostility is minimum. But that doesn't mean some won't sass you in the ass for not knowing a thing or two.
It has it's own official site too (Which I have to say, is wonderfully decorated), including a download for the latest version.
More information can be found at Youtube, Wiki, and other sites I might not know.
(Disclaimer: Sanic can't be used without Modding, sorry.)

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