During my A-placement I was teaching a lesson about 3-D drawings. For most of the students it is very difficult to draw three dimensional. So I used an activity provided by NCTM where the students can build three dimensional objects on the computer. This activity really helped them to visualize different views of 3D objects. The activity can be found at:

http://illuminations.nctm.org/

This website is really usefull since it provides computer based activities and lesson plans.