Here are some links to various software and supplemental materials.  I may add more later.

Planarity game

The applet on this page generates a random planar graph.  The challenge is to drag the vertices so that there are no edges that cross.

Kruskal’s algorithm

An animated demo of Kruskal’s algorithm to find minimal weight spanning trees.

Sage

Sage is open-source mathematical software.  It knows how to do a lot of graph theory.  You can download and install it on your own computer (beware, it’s big) or you can run it “in the cloud“.

Voronoi diagrams

This page has an applet that generates Voronoi diagrams (it seems to only work on some browsers)