I was looking at this a while ago: SmartFox Server.
I don't have the first-hand experience, but it seems promising if it delivers that it advertises.
We use ElectroServer at work (basically the same thing: Java based plugin server) and it gets the job done pretty well. Some people might shy away from that kind of server because it has to be C++ or nothing. But even for real time games the performance lost by it not being native code is offset by the safety of Java. You may lose your sanity dealing with Java though. Of course we aren't dealing with WoW type numbers so things might be a bit different at that scale.
ElectroServer and SmartFox are both around $5k for a unlimited license. Certainly not cheap, but it would take more then two weeks for me to build all the services they offer and be reasonably bug free. Both have a fully functional limited use license (~100 users) for free which is going to be more then enough for most hobby/indie games.
If you want to roll your own and don't mind being not native then Jetty or Node.js might be something to look at. Since they handle all the really low level network stuff you can just deal with the actual server logic.
I agree with tstrimple, with what you want you could probably even get away with posting data to some PHP scripts on a hosted domain.