this is the one my folks bought for DS when he was 4. It is rugged and works well. Easy to focus, top or bottom lit, cool slide samples included, tilts so little people can get a good look without climbing. It also stood up to his younger sister who started dragging it around the house at age 2 lol DS is almost 9 now and you'd never know the abuse this took over the years!! Both children have enjoyed using it over the years.

http://store.discovery.com/detail.php?p=86171&ecid=AFF-7973256
sold out here but good pix and reviews.