You could always use WCF to host an endpoint within the service and expose it that way.
The only other options I can think of would involve having some file that a web app could write to and the service could read from, or a database that's written to by the web site and read from the service. None of those are as elegant as simply exposing a communication endpoint via WCF.
Edit - Added
Specifically, I was thinking of exposing this as an HTTP endpoint and interfacing directly with the browser the way you would a web service.
However, there's no reason you couldn't have a traditional Asp.Net application set up to communicate with the service as long as the service hosts a communication endpoint.