GCompris is an educational software suite with numerous activities for children aged 2 to 10. Some of the activities are game-orientated, but nonetheless still educational. These include computer discovery (keyboard, mouse, different mouse gestures), algebra (table memory, enumeration, double entry table, mirror image), science (the canal lock, the water cycle, the submarine, electric simulation), geography (place the country on the map), games (chess, memory, connect 4, oware, sudoku), reading practice, and others (learn to tell time, puzzles of famous paintings, vector drawing, cartoon making, etc.). It currently offers in excess of 100 activities, and more are being developed.
Superficially, Dominoes on Acid is a solitaire variant of Dominoes with weird colorful tiles (and this view is enough to play the game). But on a deeper level it is a GUI for natural deduction. Every completed domino is equivalent to a proof of a tautology in classical propositional logic. The game can be played online as an applet or offline as an application. It is built on a simple isometric engine that can be used independent of the game.
Asymptopia Crossword Builder (Math/Text) is an application that runs in any modern Web browser, but does not require an Internet connection (if it is installed locally). The application was designed to allow teachers to quickly produce well-designed, challenging activities. The application features a configuration panel where the user can either enter their own word-hint pairs, or generate math-crossword problems via a single button. The math functionality generates word:hint pairs like: "four:7/x-3/8=11/8", where the hint corresponds to the problem, and the student would write the letters "four" in the corresponding boxes of the puzzle.
Sudoku Sensei is not a Sudoku solver, but an "explainer". It can list the logical steps taken. It can generate new Sudokus and rank their difficulty. It supports not only 9x9 grids, but MxN, samurai, or any layout you can describe with the board editor. It has a GUI, manual, tutorial, and examples.