Geometer's Sketchpad:

Introductory labs   (on the Math Forum website). 

Skechpad sketches of special quadrilaterals: 

Click on the name to download a sketchpad sketch of the quadrilateral (this will work on any computer that has sketchpad 3 installed).

1.  Rectangle 

2. Square

3. Rhombus

4. Kite

5. Trapezoid

6. General quadrilateral

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Some websites

A web site that reviews the basic logical connectives: 

A nice site for tessellations based on triangles, quads, or hexagons: http://www.shodor.org/interactivate/activities/tessellate/index.htm

Mathleague.com - a useful source of basic definitions