Parameter ranges are sometimes too restricted -- most of the applets cannot show limiting cases. There are no instructor resources to allow others to quickly achieve the targeted learning goals. Most items do not need it, but some would be more useful to instructors if this were available.
General Comments on Quality: Overall, these applets are of excellent quality, from both a physics and programming point of view. Some of the simulations are a bit esoteric, but most deal with topics commonly covered in an intermediate classical mechanics course, though most are accessable to better students in the introductory course. Physical parameters are easily modified. The applets show qualitative aspects of motion, but most also have quantitative data that could be used by students to check an analytic assignment. Comments specific to particular applets are given below:
Block on Inclined Plane - Although somewhat esoteric and of limited use, it does clearly show vector acceleration relationships and gives a good impression of the motion from different frames of reference.
Collisions - A very nice applet that shows two-dimensional hard-sphere collisions in the lab frame,
CM frame, or both. The user can vary the impact parameter, the mass ratio, and elastic parameter. Any change to these parameters immediately updates the momentum vector diagram, even without running through the simulation. There is linked page of equations, which is totally confusing because of the large number of equations shown (nearly all of which are linearly dependent).
Center-of-Mass - Shows the trajectory of each mass and the center of mass of a dumbell in projectile motion. The initial CM speed, velocity, rotational speed, and angle are controllable. Changes in parameters instantly update the paths. This applet gives a good sense of individual particle and center of mass motion.