TopCoder

TopCoder is the longest-running competitive programming platform, with over 1.6 million members competing in algorithm challenges. It hosts regular Single Round Matches and Marathon competitions where participants solve problems against the clock and against each other. The community publishes editorials explaining optimal solutions after each match, and many top programmers got their start here.

Suited for developers who thrive on competition and want to test their skills against a global field. The problem difficulty scales widely, so everyone from intermediates to world-class algorithmic thinkers can find challenges. It's less about learning fundamentals and more about pushing the limits of what you already know.