An interactive 100 square. You can choose to start the square with either 0 or 1.
http://www.hellam.net/maths2000/100square.html

You can also change the 'width' of the 100 board using this link:
http://www.hellam.net/maths2000/nsquare.html

Here is a different 100 square. Clink on the IDEAS button to see how you might use this tool.
http://www.amblesideprimary.com/ambleweb/mentalmaths/countersquare.html