If you prefer to use a webmail interface such as those provided by Gmail, Hotmail or Yahoo! Mail, you probably know that you cannot really secure your data directly when you are using those services. The majority of popular webmail services do not support email encryption for instance which would protect the content of messages from being read by automated tools and anyone else with access.
Mailvelope is a free browser extension for Google Chrome and Mozilla Firefox that introduces OpenPGP encryption to webmail services that you may be using. The extension ships with support for Gmail, Yahoo! Mail, Outlook and GMX by default, and options to integrate other web-based email providers as well.