The Factor Game by Ricky Carter, Ihor Charischak
Two players (or teams) compete for the highest score by picking numbers from a screen which are then added to their score. There is a catch though. Whenever a player picks a number, the total of every factor of that number still showing on the screen is added to the opponent's score.  For example, if A chooses 25, then B gets 1 and 5 (since they are factors of 25). At this point A has 25 points and B has 6.  And now it is B's turn to go. The chosen numbers are removed from the screen.
[Factor Game]
NCTM Version

Paper Pool  - NCTM
This activity simulates the path of a billiard ball on a billiard table. You choose the dimensions of the by moving two sliders. the ball will travel at 45 degree angle bounce around the table until it ends up in one of the corners. See if you can predict how many bounces and in what corner the ball ends up.
[Billiards]
Paper Pool

Spiros
Three sliders determine three numbers. The turtle uses these numbers to draw a Whumpus, Whimsy, or a Gloop. Can you predict which shape the turtle will draw from knowing the three numbers?
[Spiros]

Rolling Dice Microworld by Robert Berkman
Watch the graphs unfold as you roll 1, 2 or 3 dice. Does the law of large numbers give you a good idea of what outcome will prevail?  You will find ideas for using this microworld in Robert's and Ihor's article entitled "Looking at Random Events with Logo Software" in the Jan/March, 1995 issue of Mathematics Teaching in the Middle School (NCTM)
[Rolling Dice]

Family Fractions
It's time for all 25 fractions in Number Town to return to their “home” color pad. Your task is to move each of them to their respective pads. You are successful if clicking on the fraction (1) switches its form from fraction to picture ID (or visa versa) and not cause the fraction to "run away" from the pad.
[Family Fractions]