Venn diagrams online
This diagram, from a DailyKos diary, was made with this online applet. Unfortunately, it is impossible sometimes to get the areas proportional to your data:
The applet draws Venn Diagrams with 3 circles. It attempts to make each of the zones (the set intersections) proportional to the population (value) assigned to the zone. Note that, in general, the areas will be approximate, as an ideal solution is generally not possible using circles.