StayFocusd

StayFocusd increases your productivity by limiting the amount of time that you can spend on time-wasting websites.

You sit down at the computer, and you swear you'll be productive. Next thing you know, it's twelve hours later. You've checked your email, updated your Facebook status, browsed the trending topics on Twitter, read your RSS feeds, looked up your favorite band on Wikipedia, vanity googled yourself, cyber-stalked your ex, looked at all your high-school crushes' Facebook photos, and lost a week's pay playing online poker.

What you haven't done is WORK.

It is highly configurable, allowing you to block or allow entire sites, specific subdomains, specific paths, specific pages, even specific in-page content (videos, games, images, forms, etc).