One of the earlier entries you might not have heard about before is Sonic Kart 3DX (move aside, Sonic Drift). This was another portable outing, but it was a 2005 mobile game designed exclusively for Sega's "Sonic Cafe" service, which was only available in Japan.
As a result, most people (and even Sonic diehards) in the west didn't know about it. Now, out of the blue, 18 minutes of recorded HDMI footage has popped up on a Japanese YouTube channel.
Sure, it might not compete with the Sonic racing games we have nowadays, but it's always nice to see how far Sonic has come.