"If you use Outlook you have reason to rejoice, because there is a wide range of free add-ons that greatly enhance Outlook’s functionality and add considerably to it’s user experience.
Note that all add-ons listed below are linked to the original posts that have more details about them.
A number of Google related add-on's which I found interesting. Also some bellwethers (Xobni, etc) and some that I've not seen before... If you live in Outlook, this looks like a "must browse"