Problem of the Week Archive
This is a great resource: an archive of Problems of the Week from MathCounts.org.
MathCounts is a national math enrichment organization that, among other endeavors, runs a middle school math competition every year. Their “Problem of the Week” is typically aimed at middle school students, but these problems may also be of interest to high school students and teachers.
There are over 10 years of math problems in the above archive. The problems range in scope from basic “Math Team” style questions to thematic questions based on current events, dates, and holidays.