MathPro Online Reference to Mathematical Problems
Here are some other sites you might find useful:
- Olympiad and Putnam problems, maintained by Kiran Kedlaya at "Math Competitions Archive"
- Word Problems
- North Carolina Math Contest
- University of South Carolina Math Contest
- Rice University Math Contest
- Sample problems from the Harvard-MIT Math Tournament
- Lehigh University High School Math Contest
- International Math Talent Search
- Wisconsin Math Talent Search
- Quantum CyberTeaser Archive
- Furman University Wylie Mathematics Tournament
- University of Illinois Undergraduate Math Contests
- ABACUS International Math Challenge
- MathCounts homepage
- Japanese Math Challenge: 20 Story Problems Translated from Japan's Jr. High Math Tests
- Lots of useful links, as well as a set of lecture notes on number theory taylor-made for what is useful to know for math contests: