Schthack, Methuselah, Eden, Ultima, Sylverant, etc... There are quite a few to say the least.
Schthack's was the very first, so obviously has had the most publicity over the years. Eden, Ultima, and Methuselah have all been around for a while now as well. Sylverant is a bit of a late comer and sort of rolled together three already existing private servers into one, and has already had more drama befall its existence than anybody would ever care to know about.
Do keep in mind that there isn't any private server that perfectly replicates the experience of the original Sega servers. Only Methuselah makes an effort to try to actively replicate the feel of the Sega servers. All the rest of the other server's owners have made various tweaks and changes as they've seen fit to do so which has greatly altered the overall game experience. I definitely advise you to read up on the various servers before you try them so that you'll have an idea of just what it is your getting into.
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