If you've been using Facebook for a few years now you've probably liked pages you've completely forgotten about. Pages which just post absolute bullshit that fills your news feed with crap you don't want to see. But there's no way to go back and see what pages you've liked over the years.
Facebook Cleanser is a neat little tool that makes unliking pages a lot easier. You just sign in with your Facebook account and you're given a list of pages you've liked, even what date and time you liked them. You can quickly scroll through the list and unlike pages you don't want to follow anymore.