LeechBlock is often the extension others are compared to. It allows you create sets of sites to block for certain times on certain days, create time limits for those sites or both. It's quite effective to have such fine grain control over when you can access the offending sites. It's also got different methods for block from an error to redirection to another page.
Nanny for Chrome
Essentially LeechBlock for Chrome, Nanny for Chrome recently switched names from Chrome Nanny. It contains all the same features of LeechBlock but with the addition of some statistics and whitelisted pages. If you try to disable it during a period you've previously set up it will prompt you with an annoyingly long and specific string of characters you have to type perfectly to get access.
WasteNoTime is also similar to LeechBlock, with features that enable you to restrict time based on time of day and how long you can spend on each site, though it does distinguish inside and outside of work. Clicking the icon shows the clock of how many time you've got left, which is handy.
Content Advisor (IE)
While this isn't a browser extension, it was the only way we could find to build blocks into IE for certain sites. If you're using IE you may find that one of the freestanding programs are more useful as they are generally more feature-rich, and thus more effective, than outright blocking of sites (kind of like going cold turkey often leads to binging and abandoning of the endeavor).
Apparently Mac people are as easily distracted as the dog in Up because there's a slough of programs aimed at aiding concentration through blocking of sites. Concentrate ($29) is probably the most extensive program we've seen for this purpose. It works by creating activities that you specify the limits of. You can limit websites, open and close programs, set your chat status, lock viewing to a window and even change the desktop for each created activity. If your attention needs serious lock-down Concentrate is probably for you. Self Control (free) is a much simpler app that blocks mail servers and websites for a predetermined period of time. It's nearly impossible to stop once it gets started (closing, uninstalling or even restarting won't stop it) so if you think you're likely to have second thoughts it's a good option. Freedom ($10) is for both Mac and PC and shuts down your internet access for up to 8 hours.
BinarySwitch Eclipse (free) is a simple url blocker that sits in your system tray. It doesn't have time-based blocking so it's more of a gentle reminder to stop messing around, which may be all you need. DoNotDisturb ($39.95) program aimed to help internet addiction. It will block websites and programs for specified chunks of time and prompt you with an annoying string of characters to type if you try to edit it. Our only issue with it is the price.
We find these blocks for distractions work well because they remove temptation. With enough hassle our laziness works for us it's like our focus has doubled. If all-out blocking isn't for you, perhaps you can annoying yourself into productivity with Focusbar.