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:
AxCrypt is the leading open source file encryption software for Windows. It integrates seamlessly with Windows to compress, encrypt, decrypt, store, send and work with individual files.
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.
Have you looked at GNU Privacy Guard?
7-Zip encrypts files and is also the best open-source file archiver.
EncryptPad is an application for viewing and editing symmetrically encrypted text. It also provides a tool for encrypting and decrypting binary files on disk. It works on Windows, Mac and Linux and supports OpenPGP file format.
Link - http://evpo.net/encryptpad/
I am the developer by the way.