Rated 5 out of 5 stars
LeechBlock is very customizable (you can set up to block different websites for different time periods, or groups of websites for different time periods). It's also great, because if you want to give yourself an hourly "allowance" on your blocked sites (ie 4 minutes per hour for a break), you can!
There's also a lockdown function, where you can lock yourself out of your sites on a whim (like if you have a big task come up so you really need to shut yourself out when you'd normally have access). I haven't tried this, but am excited to.
Great app, love it, no problems here!