Facebook's "like" feature has been around since February, but the massive social network never provided users with a way to quickly voice their opinions going the other way. French developer Thomas Moquet took matters into his own hands by creating a cute (albeit useless) Firefox extension that adds a dislike button to Facebook, letting users who have it installed mark things they don't like.
In order to make the tool work, Moquet had to use his own servers, which keep track of every item that's disliked as well as who clicked it. Any other Facebook users who have the extension installed can then see who disliked it right next to the usual like list.
There are a few very clear downsides to this system, one being that if the dislike servers ever go down, you won't be able to see what you or others have marked as not liking. It also cannot be seen by other users who don't have the extension installed. Nonetheless, it fits in quite well with the rest of the Facebook interface, peacefully coexisting alongside the likes while adding a bit of snark.
It's worth noting Facebook's exclusion of a dislike button was under the pretense that likes were added as a quick way to replace simple one-word comments. By adding a like button the hope was both to better surface content in its news feeds, as well as cut down on throwaway comments like "this is great!" or "cool."
Facebook dislike is an experimental add-on, meaning you'll have to grab it from Mozilla's add-ons site. See also the competing Facebook Dislike Button add-on, which goes one step further and will actually send the person who's news item it is a Facebook note saying that you didn't like what they posted. Ouch.