Of course you could always try Google Voice…

After you install the experimental plugin, you’ll notice that every item in your Facebook News Feed has an easily accessible “Dislike” option sandwiched in between the “Like” and “Similar Posts” options. Once you indicate your less than favorable reaction, a thumbs down icon will appear under the post that ostentatiously calls out your dislike, with your name attached, for your Facebook friends to see (with the major caveat that they also need to have the plugin installed).

Update: http://www.dislikes.info is the latest release and it works.

via Mozilla.