SignalR "signalr/hubs" giving 404 error

Try call RouteTable.Routes.MapHubs() before RouteConfig.RegisterRoutes(RouteTable.Routes) in Global.asax.cs if you use MVC 4. It works for me.

        RouteTable.Routes.MapHubs();
        RouteConfig.RegisterRoutes(RouteTable.Routes);

It could be that you haven't added a reference to SignalR.AspNet.dll. If I recall correctly it's responsible for adding the route to /signalr/hubs.