LogicalSolver is a web app that helps you solving logic grid puzzles, also known as logicals. They regularly appear for example in the German magazine P.M.-Magazin, and recently in more and more newspapers and magazines.
LuckySweeper is a Minesweeper clone implemented as a web app. Are you also annoyed often that you cannot sweep all the mines due to the situation being ambivalent? LuckySweeper solves this problem. In case you have to guess, you will automatically be lucky.
WaveWorkshop is an interactive web application that simulates coherent waves and their interference in 2-dimensional space. You can use it as a demonstration, e. g. in a physics course.
CrossSections is Windows a program that visualizes polyhedra and their cross sections with a plane. It also features unfolding those polyhedra to form a lattice.