A couple games that've been sitting around in my browser windows:
The so-called "French Military Game" is a Java version of a fox-and-hounds kind of game played on a small chart that looks a little like a Kabbalah chart. You have three pieces; you're trying to stop the computer from moving its one piece past your pieces. This is one of those computer-learning games; after a few games, the computer should be able to consistently beat you.
Entirely unrelated: the Eyeballing Game gives you a series of dynamic diagrams in which you have to eyeball the midpoint of a line segment, or the ray that bisects an angle; you drag a point around on your screen, and let go when you think you're on target. It shows you how close you got; your total score is the average across three rounds of the series of diagrams. My best score is 3.24 (lower is better), but plenty of people regularly get scores between 2 and 3, and sometimes even lower. (Note that if your monitor's aspect ratio isn't what the game expects, you may have a hard time with this one.)