Graph.tk is described as «Online graph sketching app that can graph functions and numerically solve differential equations» and is an website in the education & reference category. There are more than 10 alternatives to Graph.tk, not only websites but also apps for a variety of platforms, including Android, iPhone, Android Tablet and iPad. The best alternative is Wolfram Alpha, which is free. Other great sites and apps similar to Graph.tk are Desmos, Google Calculator, NumWorks Graphing Calculator and PCalc.
