HTTPS Everywhere is a free Firefox extension created by the EFF which encrypts all communication between the browser and the site you are visiting. It only works with sites that support HTTPS, but it will ensure that your browser stays secure even if a site has a broken implementation of their security in place.
The extension covers a fair amount of sites in any event, including WordPress.com blogs, Twitter, Facebook, Google search etc, so much of your day to day traffic should gain from the added security layer. Nice little tool, and definitely valuable, especially if you do a fair amount of your surfing via WiFi and/or in public spaces like airports or transit lounges in general.
Sadly, many sites still include a lot of content from third party domains that is not available over HTTPS. As always, if the browser’s lock icon is broken or carries an exclamation mark, you may remain vulnerable to some adversaries that use active attacks or traffic analysis. However, the effort that would be required to eavesdrop on your browsing should still be usefully increased