Reuse file uploaded via (File|Image|Whatever)Field in another node
The FileField Sources module expands the file field widget and allows the user to select new or existing files through additional means, including  re-use files by an auto-complete textfield, attach server-side files uploaded via FTP, transfer files from a remote server (cURL required) and select existing files through the IMCE file browser.

The Media module (2.x) provides and additional file field widget which use its media browser to upload or select a (media) file.