even more than "just" a secure PW manager Rated 5 out of 5 stars
My favorite add-on since it's indispensable every day and works flawlessly. It is much more than just a password manager, it can save forms and any kind of info in secure notes.
There's also an offline version for your thumb drive and an iPhone app is in the works. Can't say enough good things about this app, have been using it for nearly 1 year now and it handles 99% of websites flawlessly.
If you don't believe that all traffic is encrypted locally and then sent through a secure (yet again encrypted) connection, just install HttpFox and see for yourself.