I found Free Rice a few months back from my teenage daughter.  I find it brain stimulating and intriguing.  Helping others while learning something yourself is a smart use of time.  There are words I've never seen or heard of but some of them I can make out fromt he multiple choice answers.  It also gives you second chances if you miss one.  I visit it about once a week and usually spend a longer amount of time than I initially intended with it.  It's definitely a favorite with me.