This app allows you to draw the graph of a function of a single real variable in both cartesian and polar coordinates.
It's mostly built for recreational purposes, though rendering polar and cartesian graphs side-by-side offers some insight into the behaviour of a function and lets you explore directly the mapping between typical polar figures such as roses or spirals and their regular cartesian counterparts.
If you're interested in a webgl-based version of this app, focusing on parametric graphs, see this.