How to get Current site URl

I'd like to know from what site of many where this webblock is used my webblock action is invoked (I try to avoid passing exta parameter via many actions in complecated logic so just check from what site I was invoked in one place). How to get site name like
There's an action in the HttpRequestHandler extension for "getRequestHeader", you can use that to get the "Hostname" header from the request.