I have a windows based one running on amazon AWS (free). Runs very well.
I also have other windows based servers and several linux that are hosted at cloudatcost com and all run well. As far as specs, it depends on what you want to do with it. Windows or linux on a couple gigs of ram and 2-4 CPUs can allow up to 10 or probably 20 players at once with no problems.
I am working on a new server at the moment that is windows 2012 r2 based. It loads 100 zones (90 static/10 dynamic) on 8 Gigs of ram and 8 CPUs. It runs very well and the load from the zones only hits 4.4 Gigs of ram in use.
Celestial
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