Virtual Cave - It is hard to go past this site if you are looking for information on different types of cave formations. Bear in mind that it is an american site and so some of the terms may differ from those that we use in Australia. Every type of formation is accompanied by photos, descriptions of how they form and other relevant information. There is a great graphical site map which is a picture of a cave with links placed on each of the formations. This would look great printed out in colour. On a down side, this site is often inaccessible.
Mineral Net - If you want to get down into the different types of minerals and crystals, not just related to caves but to the mineral world in general then this is the site to visit
University of Texas Biospeleology - Students wanting to learn more about what lives underground and why should give this excellent site a look.
Research on Glowworms in Australia - Great information from an ACKMA member, Claire Baker, on the glowworm species in Australia with a mention of the famous Glowworm cave in New Zealand.
Illinois Natural History Survey - As with the University of Texas site, I consider this an excellent site for research in speleobiology.
Biospeleology Links - This is a Canadian site that gives quite a view links to biospeleology sites around the world.
Karst Waters Institute - Students doing assignments on ecosystems and in particular how sensitive caves are should have a look at this site.