Some websites to prevent mathematical cobwebs

If you have a long break this (Northern hemisphere) summer, it's important to keep your brain ticking over mathematically so that you don't have to start up from cold when you go back to school next term. I've been on the lookout for some recreational mathematical activity on the web: even just a few minutes each day will be beneficial. I've included a partial screenshot from each site. Pattern recognition Enlarge any one of these 240 patterns. You have to work out ...