Rated 5 out of 5 stars
This does one thing beautifully - it BLOCKS sites that you don't want to be bothered with. Sometimes - even though I'm pretty safe with ABP, et al., something manages to get through. All you have to do is add the site to your BLACKLIST, and...gone (it also has whitelist, if needed). My rule is simple; an add-on should DO what it says it'll do, and do it well (and as easy as possible). BlockSite does it wonderfully.
Some have made the error thinking this add-on is like Cybersitter, et al. - it is NOT.
For that - look at those products. NOT here.
It's so YOU are not harassed by sites you've either had previous bad experience with, or, sites that you know are bad.