SoftBlock

Works well enough! Breaks when going to links with extended urls that start with the blocked address, though. It'll properly block and ask you to unblock reddit.com for example, but with just "reddit.com" in your block list, it'll give a "gpniknfkgmfphohfhbnacmcjmplbgfjk is blockedThis page has been blocked by Chrome ERR_BLOCKED_BY_CLIENT" Error and not prompt you to unlock. Still, better than most available options nowadays.

very simple, works fine for me. excellent in giving me a reminder to do usefull stuff instead of going to other sites

I wish it worked, because it is so simple and would suffice, but its broken in Chrome Version 39.0.2171.95 The block works, but the button to go to the website (after selecting a period of time) doesn't work, so you can't get to the website at all.