This is absolutely one of my favorite sites - especially since the responsive design update. The isotope search is absolutely gorgeous, along with the new format, in general. I may be a bit biased though, since it was this website that got me into web development and serious programming in general. (At that time, ptable wasn't responsive and I wanted to create a different periodic table site that had that feature). I'm glad ptable is being featured here - while I was at college I was surprised to find that not a significant majority of people knew about the site. Even for high school chemistry, it's orders of magnitude easier to use compared to the textbook or online ad-hoc element search queries.