Nanny for Chrome is a free blocking extension for Chrome aimed at older children to get them to concentrate on something other than fun … like homework. It also works for the inner child of most adults, getting you to concentrate on work and avoid play. Reviews are mixed as people who aren’t very computer-minded might have trouble setting it up. The more geeky ones love that you can block subsets (block Twitter, but allow Twitter notifications) and use blocking sets with different rules. A free extension well worth taking the time to learn how to use.