LifeHacker's Afterhours blog is featuring the OKC Predator Alert. You should not install it, because the creator has been accused of using the extension to mine people's data so that he can, well, prey on them, basically. He's actually a pretty abusive, predatory dude himself (repeatedly, in multiple incidents over a variety of years). I'm not going to go dig up the tweets from months and months ago when he was accused, but yeah, just don't, just in case.