"The National Library of Virtual Manipulatives" (Software package promoted for sale, but they also have a free version. Very nice site chock full of online applets and manipulatives for teaching math concepts to students of all ages. ALL the online manipulatives – and there are HUNDREDS of them – are free. http://nlvm.usu.edu/en/nav/vlibrary.html
CRAZY interactive website that shows relative sizes of things... From a coffee bean to a grain of salt to a human skin cell to an E. coli bacteria to an HIV virus to a carbon atom with a bunch of others thrown in for comparison. WOW!