In the EFX world, the user visits the EFX site, uploads the file, enters the receiver's email address and lets the system notify the receiver that there's a file waiting for them, via a simple email message.
Senders need to sign up, and the EFX system only allows transfers between 'internal' users (those with addresses in an approved list) and external users, never external to external.

Written in Python, using the Pylons framework.
Licensed under the GNU GPL version 3.

** NOTE: As I'm not actively developing EFX, perhaps this project might suit your needs: http://freshmeat.net/projects/fffex