I often went to the ZOJ(Zhejiang University Online Judge)site to do some exercise before I took part in the Zhejiang Province Programming Contest. Attending this activity gives me much joy and my fellows in the contest and our coach are both very kind.
Since having been graduated from the university, it is a pity that I have no opportunity to attend the ACM/ICPC now. If you are still a student in the university and love computer programming then you can have a try! Here are some sites may be useful:
The official site of ACM/ICPC http://icpc.baylor.edu/icpc/
Zhejiang Univ. Online Judge http://acm.zju.edu.cn/ (This site crashed recently,^_^)
Peking Univ. Online Judge http://acm.pku.edu.cn/
Ural State University Problem Set http://acm.timus.ru/
Some tips for Ural problem sets http://oibh.kuye.cn/ural/