Electroserver or Smartfoxserver
Electroserver rocks. It's simple, easy to use and quick to learn. Smartfoxserver on that other hand is slightly different but if you have worked on ES then working on SFS would not be difficult. SFS has an api for C#, now this is a plus for most people developing apps for windows or mac. Support for Actionscript however is there both for ES and SFS. So this would help game development for online games. Learning curve for ES is low and hence you could speed up in no time. It has got a good API and have a great documentation. And best of all, it is got a good customer support and forum. So my choice would always be on ES.