IE View is a simple Mozilla and Mozilla Firefox extension (for Microsoft Windows systems), which allows the current page or a selected link to be opened in Internet Explorer.

Adds a little panel to the page when you are looking at a book to find where you can buy the book online for cheaper.

McAfee siteadvisor tells you how safe a site is based on their tests of how much spam you will receive from the page, etc.

Lots of extensions for firefox.