If you have 24 people in a room, what is the chance that at least two of them have the same birthday? What about 30 people? 50 people? This applet simulates the problem. The answer may surprise you!
http://www-stat.stanford.edu/~susan/surprise/Birthday.html

Peg Solitaire (also known as Hi-Q) has very simple rules. Pegs (red circles in this implementation) are allowed to jump over adjacent (vertically or horizontally) pegs. The peg that has been jumped over is removed. So jumps are like captures in Checkers. To move a peg, you select it first by clicking on it once. Then you click at the square (hole) to which you want the peg moved.

http://www.cut-the-knot.org/proofs/pegsolitaire.shtml

Clockwise
Either enter a time and see it displayed on the analogue clock, or click on 'Random' and type in the time shown on the clock. There are three difficulty levels.
http://www.shodor.org/interactivate/activities/ClockWise/

Elapsed Time
Practice finding elapsed time given a starting time and an ending time.
http://www.shodor.org/interactivate/activities/ElapsedTime/

Type in the Input. The Function Machine applies the function and produces the Output. If you choose [Random], you will have to use some brainpower to figure out the hidden function.
http://www.amblesideprimary.com/ambleweb/mentalmaths/functionmachines.ht...

Linear Function Machine
With this function machine, you have to enter the correct linear function, using algebra.
http://www.shodor.org/interactivate/activities/LinearFunctMachine/

Students play a version of Connect Four, gaining the chance to place a piece on the board by answering questions about simplify fractions, converting fractions to decimals and percents, as well as algebra questions involving fractions.

http://www.shodor.org/interactivate/activities/FractionFour/

Factorise
Visualize factors through building rectangular areas on a grid. First enter all the factorizations of a number, then draw each factor set as an area on the grid. Factorize is one of the Interactivate assessment explorers.

http://www.shodor.org/interactivate/activities/Factorize/

Factorise II
Visualize factors through building rectangular areas on a grid. As you draw each factor set on the grid, the factors will be listed.

http://www.shodor.org/interactivate/activities/FactorizeTwo/

Practice estimation skills by determining the number of objects, the length of a line, or the area of a shape. Choose the problem type, or choose 'Random' to see a mixture of all three types of estimation questions.

http://www.shodor.org/interactivate/activities/Estimator/

Play a "Connect-Four" style game, earning a game piece each time you answer an estimation question correctly. This game asks the players to estimate the answer to addition, multiplication, and percentage questions. Choose difficulty level, question types, and time limit. Estimator Four is one of the Interactivate assessment games.
http://www.shodor.org/interactivate/activities/EstimatorFour/

Here is a similar interactivity that doesn't have the Connect Four interface.

Estimator Quiz
http://www.shodor.org/interactivate/activities/EstimatorQuiz/

In this interactivity, you can choose 'Counting' where the students have to estimate the number of stars.

Estimator
http://www.shodor.org/interactivate/activities/Estimator/

More or Less Estimator
Is the number of objects more or less than the given estimate?
http://www.shodor.org/interactivate/activities/MoreOrLessEstimator/

Roll or choose a number in Pascal's Triangle and then click on all entries that are multiples of the number, thereby practicing multiplication tables, investigating number patterns, and investigating fractal patterns.

http://www.shodor.org/interactivate/activities/ColoringMultiples/

Arithmetic Four is a Connect Four variation where you have to answer an arithmetic question correctly before you can play a chip. You get to choose the category, the operation, the difficulty level and the time limit.

http://www.shodor.org/interactivate/activities/ArithmeticFour/?version=1....

Arithmetic Quiz is similar but without the Connect Four interface.

http://www.shodor.org/interactivate/activities/ArithmeticQuiz/