Mailvelope is a browser addon for Chrome and Firefox compatible with OpenPGP encryption standards, it will not only encrypt your webmail messages but also read any encrypted email you receive from people using different OpenPGP encryption software like Enigmail. The addon integrates directly into the browser and it comes preconfigured for use with the following email providers: Gmail, Yahoo Mail, Outlook.com and GMX. However it can be customized to work with any other webmail service and it also supports the RoundCube email software, frequently found in hosting companies offering email services with your domain name.
When you receive an encrypted message the addon will try and find the encryption key used to cipher the message in the keyring and prompt you for a password. Anyone familiar with the public/private key encryption scheme will find this addon a very easy way to encrypt and decrypt messages, it could also be used to post encrypted messages on any forum or Facebook if you want to. Being a browser addon means that it will work on any operating system and it can be added to a portable browser.
There are other free tools to encrypt webmail messages but this is one of the few that is not specific for a service and it will work with any webamil, together with the fact that MailVelope is an open source project using compliant OpenPGP standards makes this addon worthwhile to consider for those worrying about their personal messages travelling through the Internet like a postcard.