Streaming files to the client is very easy using ASP.NET MVC 3: The following code snippet shows an exemplary controller action "Download" that streams data to the client. If the client requests this action (e.g. by using the link [YOUR_CONTROLLER]/Download) the browser will (depending on it's settings) start downloading the data or open the download dialog.
The first optional parameter mimeType (e.g. "image/jpeg" for jpeg images or "application/pdf" for pdf documents) defines the HTTP header Content-type which is used by the browser to decide how to handle the data. The second optional parameter fileDownloadName defines the name of file the data should be saved to on the client's computer.

public ActionResult Download()
  var fileStream = [...];
  var mimeType = [...]; // optional
  var fileDownloadName = [...]; // optional
  return File(fileStream, mimeType, fileDownloadName);