From Desmos we have the outstanding graphing calculator, their Facebook page (you do not need to be a Facebook user to view this) is also worth keeping an eye on. Some of the photos available make ideal starters. See the Think Fast series or perhaps Mental Maths Monday; there are also several probability problems. I think I’ll try some of the Mental Maths Monday series with my sixth form students – look at this for example!
The problem shown in the image could be used with younger students but also with older students who could look at an algebraic proof.