SecureGmail encrypts and decrypts the messages you send through Gmail. This happens to computer your and so the unencrypted text never reaches its servers Google. It is a very useful one Plugin (works only on Google Chrome and Apple's Safari) if you want to be sure that only the final recipient will read your email and not companies, governments, etc.).
The project is very reliable as it is open source. Community involvement guarantees transparency in the code but also helps identify potential bugs. The whole project is in GitHub for anyone who wants to control the code, and according to the developers every tweak you have to suggest is always welcome.
How safe is it?
SecureGmail uses symmetric encryption to encrypt and decrypt each message. The password decided by the user is also known to the recipient.
It's only as secure as your password (short passwords provide minimal better safety). Remember, you should never share passwords via email or IM. Use as long as you like for encryption.
SecureGmail uses an open source JS Cryptography library available from Stanford here as well as in Github.
The plugin is compatible with any Gmail account and you can use it in multiple accounts at once. Works well in personal Gmail accounts or accounts provided by your company, school, or organization through Google Apps.
Download the Chrome plugin: