http://www.srb2.org/history/historic-downloads/

It's basically what started the whole Sonic fangame craze. Was made by Johnny Wallbank, who went by the monkier Sonikku.

Brief history, taken from the website:

Meanwhile, over in England at Sonikku’s school, many kids couldn’t understand why he liked
Sonic. "Sonic’s stinks, and Sega’s dead! No new games are coming out!", they
taunted. He took it to heart, and armed with MSPaint and Klik N’ Play, began work on a
game to prove them wrong, which later become known as Sonic Robo Blast.

Atrocious graphics, badly written Sonic MIDIs, and bugs everywhere. What's not to like? In particular, Sonic's sprite is an absolute classic. There's also a bug known as "killingdead" which means that you sometimes lose your rings even when you kill an enemy with your jump.

Latest version was built using The Games Factory, for those who are on 64-bit systems and/or using Wine. You'll need to download cncs32.dll, if you don't already have it.