MathCounts - the premier middle school math contest.

ARML (American Regions Mathematics League) - an annual national math contest attended by many top names.

Internet Math Contests - 2 popular internet-based math contests, 1 for fun and 1 for perhaps a bit more.

In Southern California - 2 college sponsored math contests, 1 by mail, 1 on-site.

Putnam - THE math contest for college undergraduates, and a good reason why middle-high school contests matter!

Other Math Contests - 3 more relevant contests, 1 of them for college undergraduates.