I'm haven't looked for a HTTP server recently, the only one I know is Apache Tomcat which is free. There are however also multiplayer functionalities in game engines as well. I know the Unity engine has it (and I bet the Unreal engine has to have it too). The Unity engine and the Unreal engine aren't free in case you make substantial money with your games. But they are free for hobbyists. You'll want to check out your different options of game engines considering you want to do multiplayer.