These three interactives from Shodor are a good introduction to Venn diagrams:
Venn Diagrams, Shape Sorter and Triple Venn Diagram Shape Sorter
With the Triple Venn Diagram Shape Sorter you can either set the rules or guess the rules by selecting the appropriate botton:
(Note the details of Shodor Interactivate here and all the activities here.)
Some excellent activities are available from the Illuminations site.
The Shape Sorter allows exploration of geometric properties of shapes.
Select Instructions and Exploration for clear information on how to use the resource.
From the National Library of Virtual Manipulatives, this Venn Diagrams applet allows exploration of set operations such as union and intersection.
WolframAlpha can also be used.
The slideshow here shows several examples.