I need to e-mail a file to someone with whom I have already negotiated a password. What are reputable, reliable and open-source tool with simple UI? An example for disk (as opposed to file) encryption is TrueCrypt. Platform is windows but available linux port is a plus.
AxCrypt seems to be a good place to start from. Quoting its webpage:
If you want to add some nice spice into it, and you're exchanging these files on a regular basis, create an encrypted disk within a file with TrueCrypt and share it with your friend using dropbox. It will give you a flexible, seamless directory structure which is automatically encrypted and synchronized.