The United States of America Computing Olympiad (USACO) is a computer programming competition aimed primarily at secondary school students in the United States. Participants of the USACO submit programs in one of four languages, C, C++, Java, and Pascal, to various competitions held throughout the year. (from Wikipedia)
The USACO contains several training pages on its website which are designed to develop one's skills in programming solutions to difficult and varied algorithmic problems at one's own pace. In addition to around 100 problems, there are texts on programming techniques such as greedy algorithms, dynamic programming, shortest path, and many others. Enthusiasts find the training pages so useful that people from other countries use them to prepare for their own national Olympiads; those from other countries in fact outnumber U.S. participants. (from Wikipedia)
Written 2011 by Nathan Li, nasiless[at]gmail.com or Github. Released under the MIT license.