Wolfram Alpha is an online query system to answer any question. I only use it sometimes to query for simple mathematical objects without having to resort to the paid Mathematica software.

Plot: The plot query can be used to plot functions of one and two variables as 2D and 3D graphs respectively. For example, plot sin x plots a 2D graph of that function. Similarly, plot sin x cos y plots the 3D graph of that function. The 3D graphs are quite beautifully plotted with colors and contour lines, but are only shown from two views (angled and top). If you need to interactively move it around to observe it, you need to pay for a Pro version.