1.uva.onlinejudge.org/
UVa Online Judge
2.icpc.baylor.edu/past/icpc2007/Re ... eport.html
Official International Collegiate Programming Contest (ACM) site, 2007 results (You can change 2007 in URL to 2006, 2005, etc.)
3.oi.edu.pl/
Polish Olympiad in Informatics
4.ipsc.ksp.sk/
Internet Problem Solving Contest - Comenius University
5.informatik.uni-ulm.de/acm/
ACM Programming Contest at the University of Ulm
6.hsin.hr/2004
hsin.hr/coci
Crotian Olympiad in Informatics 2004 (You can change 2004 in URL to 2001, 2002, etc.)
7.mathworld.wolfram.com/
Mathworld, many useful mathematical resources
8.forums.topcoder.com/
apps.topcoder.com/wiki/display/t ... t+Analysis
Topcoder Forums & wiki, many programming resources
9.plg1.cs.uwaterloo.ca/~acm00/
Waterloo Programming Contest official web page.
10.web.mit.edu/acmicpc/www/
The MIT Programming Contest
11.spoj.pl/forum/viewforum.php?f=40
DASM Programming League (hosted in SPOJ) discuss & solutions.
12.ace.delos.com/DEC08
Usaco Contest (December 2008) Results and solutions.(You can change DEC08 to MAR07, Jan09, etc.)
13.cemc.uwaterloo.ca/contests/past_contests.html
Canadian Computing Competition Official Website.
14.livearchive.onlinejudge.org/
ACM-ICPC past contests' problemset archive
15.ideone.com/
Online program-testing website, using your own program and input file
Any other good site links are welcome.