Interactive Data Structure Visualizations (IDSV)
http://nova.umuc.edu/~jarc/idsv/
Created by Duane J. Jarc, George Washington University (now at University of Maryland University College). Created in 1997, updated in 2005. A collection of applets with a key innovation that it incorporate pop-up windows to ask questions of the students.
Sourcecode is not public, but can be obtained on request to the author (Duane Jarc <jarc@nova.umuc.edu>).