I mainly use the DuckDuckGo search engine. DuckDuckGo is also available as a Tor Hidden Service at:

http://3g2upl4pq6kufc4m.onion/

Using this .onion address means you never leave the Tor network and will, of course, require you to have Tor installed to be able to get at it at all.

I've made a simple search plugin for Firefox that uses this hidden service. No fancy packaging yet. You will have to copy it to the right place yourself. Get it at http://hack.org/mc/projects/duckduckgo-tor/.

UPDATE: If you go to http://3g2upl4pq6kufc4m.onion/html/ and go to the search bar menu you will see an extra menu option:

Add "DuckDuckGo (HTML)"

This will do the same thing as installing my plugin, except for the name.