Matt has attended the Boston area Math Circle for the last two years, since grade 6. It's the best thing that's happened to him in math. Its website is: http://www.themathcircle.org
The Boston area Math Circle is non-competitive; it does not prepare students for math competitions of any kind. The Bay Area Math Circle, however, does prepare students for competitions, all the way to the Olympiads. I would assume that it therefore appeals to a different kind of students. The Boston-area Math Circle is not connected to a single school or district, but attracts students from all over the Boston area.
There are weekday classes (one hour) and sunday morning sessions (3 hours divided into 3 segments). There is no homework.
The topics covered by the Boston area Math Circle are outside the regular curriculum. Topics have ranged from "Are There Numbers Between Numbers?", The Euclidean Algorithm, Linear Functions (for 5-7 years olds) to Cantorian Set Theory, Random Walks, Min/Max Problems (for 7-9 and 9-11 years olds) to Pythagorean Triples, Concurrency and Collinearity (10-11 years olds, no algebra) to Projective Geometry, Complex Analysis, Combinatorial Geometry (15-17 years olds with good algebra and geometry). The list of topics is quite long. Typically, one topic is covered in ten one-hour sessions, coinciding with local universities' academic calendars.
Some of the problems appear in James Tanton's Solve This (MAA, 2001). Tanton is one of the three Math Circle organizers.