A free clipart collection from Dorling Kindersley (DK) is a useful free resource for school and other educational and class-based projects. They include a wide variety of images, from flags and historical imagery, to the human body and animals. Some of the categories include: Ancient Eqypt, Space, Flight, Human Body, Religion, Music, Earth, Nature, Science, Vikings, Birds, Maps, etc.
http://www.dorlingkindersley-uk.co.uk/static/cs/uk/11/clipart/home.html