DD did the Noetic Learning Math Contest last year:
http://www.noetic-learning.com/We're planning to do it again. 2 local math centers hosted it last year, and at least 1 of them didn't require us to have taken their classes to enroll in the contest.
That's interesting. The trouble (for us, as homeschoolers) is it looks like a school-based/team contest. The ones I mentioned are individual, with no school connection required.
It may depend on the organization hosting the event near you. It's a paper-based contest so somebody has to organize it and host it and your student has to show up in person. The local organization we enrolled through offers after-school math classes at some local elementary schools, and also after-school and weekend math classes at the community center. But we did not have to be enrolled in their classes to register for the contest through them. I just signed up on the local program's website, plunked down my $25 (or whatever it was), and made sure DD showed up at the right place at the appointed time. She was considered a member of their "team" by virtue of having signed up through their school.
The same place we did the Noetic contest through also hosts the Kangaroo contest, but we missed it last year.