Blockr is a webapp and Chrome Extension that forces you to work before it allows you access to the internet. It helps you set goals, waits for you to submit them, and allots a specific amount of browsing time based on the achievement of those goals. For example, I set my first goal as 250 words and am using Blockr to write this post.
It's a clever way to limit yourself, and it's nice that everything is integrated into one place so you're forced to work in a given area. If you're a coder, you can even set code-specific goals for better tracking. The downside is that ultimately, if you really want to focus, you ultimately have to make that effort yourself rather than rely on an extension. It's not hard to disable it and you can always just open another browser.
It's definitely a useful tool, especially for keeping track of how much web browsing time you use, but you still have to motivate yourself enough to get things done
If you want to give it a try, just head over to the site and sign up. Be sure to sign up and get started before downloading the Chrome extension. I broke the rules and installed the extension first. This made Blockr think I was an anonymous user and all sorts of errors followed. Clearly it's all about you following the rules, even in the sign-up process!