This somewhat unexciting applet was the final project for the entry-level Computer Science class at BYU the first semester I was a TA for it. Basically it does sumations, products, and factorials, and keeps a log of events. The students have 3 hours to complete it, which turns out to be a challenge, but I did this one in 45 minutes while simultaneously passing off parts of it for them.
Report bugs and give suggestions here.
These applets were developed by Matthew Reeder. Please get my permission before using these applets on your own site.
(And trust me, it will be easier if I help you use them :-p)