Area Quiz by Wasabi Applications is a series of geometry problems. Just find the area of the shaded region. I really like these. The diagrams are crisp and clear. The problems themselves can be quite tricky.
There are approximately 120 levels and I've done 109. The program allows the user to draw on the figure, but there's really not a lot of space for that. That's the only minor complaint.
High praise: I'm going to copy some of these problems for my students.