1. The ACM-ICPC International Collegiate Programming Contest. http://icpc.baylor.edu/icpc.
2. Programming Contest Control System (PC²), California State University, Sacramento, U.S.A. http://www.ecs.csus.edu/pc2/.
3. Ural University Problem Set Archive. http://acm.timus.ru/en.
4. Online Judge from the Universidad de Valladolid, Spain. http://acm.uva.es/problemset.
5. Open Source Initiative. http://www.opensource.org/.