Thomas Wolf, who is a professor in the Brock University Mathematics Department, has created a wonderful series of mathematics contests for elementary-school students: the Caribou Mathematics Competition.
Contests are available at each of three Grade levels: Grades 3/4, Grades 5/6, and Grades 7/8. The next contest date is 16 February 2011, so if you are an interested parent or teacher of a child in one of these grade levels, you should definitely check this out soon. Currently the contests are officially available only to students attending school in Canada, but there are plans to take the competition to a wider audience. Registering for a contest is easy, students find the contests fun, and teachers say they are a useful complement to their teaching.
There is a very useful set of online resources (including all past contests) available that provide excellent training and practice. These are available free to anyone in the world.
The purpose of the Caribou contests and the online practice material is to promote the study and enjoyment of mathematics among elementary-school students. And the fact that everything involving Caribou is absolutely free, both to students and their schools, admirably supports these goals.