Phillips Exeter, one of the most prestigious U.S. boarding schools, is having a middle school math tournament on Jan 28 2012, and this year it is also organizing an online tournament
http://csserver.exeter.edu/~mathclub/emc2/ .

Problem sets for their math courses, from algebra through multivariable calculus, are at http://www.exeter.edu/academics/6539.aspx .

Thanks very much! I'm always looking for new sources of math problems and tips, and this one looks great.