Chrome/Firefox: It's no secret that email is grossly insecure. If you want a little privacy in your inbox, the easiest way to do it is to encrypt your messages. Mailvelope offers free, OpenPGP encryption for most popular webmail services that is easy to configure and a breeze to use.
Mailvelope is in beta, but both the Chrome and Firefox extensions worked well in our tests. Gmail/Google Apps, Outlook, Yahoo! and GMX are all supported, and the app can be configured to support others. After installing the extension, you can generate your own public and private keys to use to encrypt messages in the extension's preferences. Once your keys are ready, the next time you compose a message, you'll see a lock in the compose window that you can click to encrypt your message. You can even use multiple keys for multiple recipients.
When you get an encrypted message, the process works in reverse. You'll see the encrypted message with a lock overlay over it, and you can click it to enter your key password. Mailvelope will search your saved keys to find the one needed and decrypt the message for you.