Is there a browser add-on to filter Google search results?

I can't tell if you're kidding or not.

Personal Blocklist does exactly what you need, and it was released yesterday.

The personal blocklist extension will transmit to Google the patterns that you choose to block. When you choose to block or unblock a pattern, the extension will also transmit to Google the URL of the web page on which the blocked or unblocked search results are displayed. You agree that Google may freely use this information to improve our products and services.

Here's a Greasemonkey script: