DuckDuckGo

To add DuckDuckGo to Internet Explorer, simply go to the homepage and click Add to IE at the bottom. If you'd like to add the encrypted (SSL) version, use the HTTPS homepage when doing so. To set DuckDuckGo as your homepage, do the same but click the Set homepage link instead.

These (non-Company) extensions also use DuckDuckGo:

• Search plugin by Woody Pewitt - available in the IE Add-ons Gallery.
• DDG + Google Suggest by Nathan Friedly (open source) - adds Google auto-complete.