blocking wbsites

Leech Block is a good extension for browsers

 

k9 web protection works really well. You can have a friend set it up for you, or use a random password.

 

@19m,
If you don't mind paying a little, Covenant Eyes is really good. (Note that the organization is Christian, if that matters to you.) It also can email internet usage reports to your accountability partner(s).
k9 is a lot more customizable (and free).
I've also used Qustodio. It's more supposed to be parental control software. But if you have a trusted friend (or accountability partner) set it up, I've found it works pretty well (even the free version).