Daytum is a web application whose primary purpose, I think, is to make it easy for users to input personal data (exercise, food) etc. over time and see it displayed in a variety of ways.
I guess some people would find that useful. However, I personally was attracted by the ease in which you can input any kind of data you want and then have it displayed in so many different kinds of charts and graphs. That could make it a useful tool in class, similar to other sites listed in The Best Tools To Make Simple Graphs Online.
You still have to request an invitation to try it out, but it appears they will be opening it up to the public very soon.