I discovered the "verbatim" option under the "More search tools" link on the left hand side recently.

http://support.google.com/websearch/bin/answer.py?hl=en&...

It's not as convenient as an extension or a script, of course.

It should be easy enough to write an extension or even a bookmarklet for this. You just have to add &tbs=li:1 to the end of the URL to enable "verbatim" mode.

Not an extension, but:

http://finderr.org

Disclaimer: I made that, but I gain nothing from its usage and don't collect any private information (like the search terms).

I have one such extension on my PC (but I am on my phone now, sorry. )