SignalR is like Socket.IO in that it supports transport negotiation/fallback. It is a framework and not a server, so you need to host it on a server of some sort. We have hosts for ASP.NET, OWIN (e.g. Kayak) and self-host, so you can run it in your own process easily, e.g. a Windows service.
SignalR has supported clients for browsers (JS), .NET, Windows Phone 7 and Silverlight. There are also contributed clients for things like iOS, Mono Touch, etc.
SignalR will give you a much higher level API than raw sockets which is its big advantage, allowing you to do things like “RPC” from server to clients in a broadcast (or targeted) fashion.