Stayfree is a company that produces several different useful apps/browser-extensions.
For website-blocking, the reasons I like their products are:
You can set per-website time limits (for example, 15 minutes a day). Time limits work for me better than complete bans, because it ends up making me approach problem websites with the mindset of "let me get in, read a few things, and then get out ASAP". In other words, it encourages me to read just the most important pieces of content, instead of thinking of the site as a "bottomless pit" of content that I can [ab]use as an endless distraction.
At the end of the day, it provides metrics on your time use, which although not strictly necessary, is still a useful feature sometimes.