3 links
Bay Area Math Circle
mathcircle.berkeley.edu
Features The Bay Area Mathematical Olympiad (BAMO) and weekly program for San Francisco Bay Area high and middle school students.
Pomona College / University of Wisconsin High School Math Talent Search
math.pomona.edu
Year around contest that tests talent in problem solving.
Stanford Math Tournament
stanford.edu
Annual math event sponsored by the Stanford Mathematics Department. Site contains results, past tests and solutions.