在后台运行大文件，从浏览器启动

Is there any reasonable method to allow users of a webapp to download large files? I'm looking for something other than the browser's built-in download dialog - the requirements are that the user initiates the download from the browser and then some other application takes over, downloads the file in background and doesn't exit when the browser is closed. It might possibly work over http, ftp or even bittorrent. Platform independence would be a nice thing to have but I'm mostly concerned with Windows.Is there any reasonable method to allow users o