I always wanted to try this game when I was a kid. When checking out reviews on Steam I noticed someone mention that Sega was giving it away for free for their 60th anniversary (a few years ago), and that website is still up and running. Long story short, I tried it and it works. Replacing the localhost part of the URL was necessary as the screenshot mentions.
To me it has a similar feel to some of the later Sonic games, but without the fear of falling off the level. I recommend checking out a how to play guide because it just jumps in without explaining.
While I am at it :D (Edit: broken links corrected)