http://www.amblesideprimary.com/ambleweb/mentalmaths/testtest.html

Test your multiplication facts. There are three levels: easy, medium and hard

A lovely hundred board from Ambleside Primary. Click on the [Ideas] tab to get some suggestions on how it might be used.
http://www.amblesideprimary.com/ambleweb/mentalmaths/countersquare.html

Here is another. You can drag the yellow bars to cover numbers:
http://www.amblesideprimary.com/ambleweb/mentalmaths/interactivemultipli...

Here is a 1 to 100 board that you can write on, using the mouse:
http://www.amblesideprimary.com/ambleweb/mentalmaths/scribblesquare.html

http://mathplotter.lawrenceville.org/mathplotter/mathPage/riemann.htm

A lovely Geogebra java applet from Miguel Bayona, The Lawrenceville School, Lawrenceville, NJ. It demonstrates the lower sum, upper sum, left sum, right sum, midpoint sum and trapezoidal sum for a function of your choosing. Loading the applet can take a while, so be patient.

The Tower of Hanoi puzzle is an engaging and challenging problem for middle school students and an application of exponential functions. It also is the setting for an elegant proof by mathematical induction that the minimum number of moves M to move n disks from one disk to another is given by M = 2^n - 1.

Here is a Java implementation of the same puzzle.
http://nlvm.usu.edu/en/nav/frames_asid_118_g_3_t_2.html?open=instruction...

http://www.babynamewizard.com/namevoyager/lnv0105.html
The Baby Name Wizard is a fascinating website that graphically displays the number of babies with different names over the years. You can easily track the popularity of different names. What do you think was the most common boy's name and most common girl's name in 1990?

Note that the website takes some time to appear as it has to download a lot of data. Be patient!