The International Slide Rule Museum is a Not-for-Profit commercial-free Museum Dedicated to the Students, Educators, Scientists and Engineers of the Past and Those Still Present, and to promote the lost art of Numeracy by providing resources and slide rules for education and other historic institutions.

This is a great resource site for teachers and students who don't understand place value or number lines and as boring as it may sound it is quite amazing what has been achieved and learn't through the simple but amazing devices.  Man was put on the moon with one of these puppies and nearly every major landmark of the last 500 years was calculated and designed on one. 

Learn some lost skills that will still amaze your students. Check it out here.