Wooden 1" cubes (100)
This is an extremely versatile tool that you can use for activities starting at age 3 and up.
At a very early age, kids can sort by color, count them using a variety of counting strategies, and explore measurement (particularly in inches!). As they grow older, they can explore ideas such as area and surface area. They can use these to model situations posed in complex math problems.
And of course, kids love stacking them.
MathLink Cubes (100)
Another versatile learning tool that is fun to play with. These are especially good for more advanced counting activities in which you want your child to practice strategically grouping objects while counting, place value, pattern games, and exploration of volume and surface area.
They don't make as satisfying noise on the tabletop as the wooden cubes, though.