LeechBlock is one of the cornerstones of my online life; it is an add-on that lets me set up a list of "time-wasting" websites and forces me to stop using them after some time has elapsed. It's one of the main add-ons that's keeping me on Firefox (rather than Chrome or Opera, which I really do like).
StayFocused tries to bring a very similar feature set to Chrome. I looked at this extension a couple of months ago, and a second look this morning shows that, with many new features added, the developer has been hard at work.
In addition to specifying which sites to block, you can now configure active days and hours (meaning, when to apply the blocking), blocked sites, and allowed sites (for white-list functionality). There's also a "nuclear option" for blocking access to all but the white list (or all including the white list, which would render your browser useless), and there's a "require challenge" option that forces you to type in a random string before you can change the options.
If any of these seem familiar to you, that's because StayFocused seems to have taken a page right out of LeechBlock's book. LeechBlock has all of these features and many more. For example, LeechBlock lets you configure five different blocking sets and control the length of the string. With LeechBlock, I can configure it to let me access my time-wasting websites for "5 minutes every hour."
Don't get me wrong, StayFocused is taking steps in the right direction. If you're serious about blocking distracting websites, though, it doesn't come close to the versatility and simplicity that LeechBlock offers. Also, I wish that StayFocused would give a bit of credit where credit is due; I could find no mention of LeechBlock in the FAQ or elsewhere.