How many times have you said, "I've attached the file below," and then failed to actually attach the file? It happens to even the best of us.
AttachCheck is a program that seeks to end this forever. AttachCheck looks through all outgoing mail. If it expects an attachment and doesn't see one, it refuses to send it until you confirm that you really want to send the message sans attachment.
AttachCheck is copyright 2005 by Benjamin Mako Hill and is distributed under the GNU General Public License as Free Software.