- Association for Computing Machinery. The academic and professional organization that sponsors the ACM-ICPC programming contest.
- ACM International Collegiate Programming Contest. A major annual programming contest for university students.
- A competitive programming platform that offers regular programming contests and a library of problems from previous contests. It is currently the most popular such platform, and attracts top competitive programmers from around the world.
- Project Euler
- An online judge that specializes in mathematical programming puzzles. Unlike other online judges, it doesn't accept source code as a solution. Instead, contestants run programs locally, and submit only the program output.
- Topcoder started as an algorithmic competitive programming platform, and later branched out into crowdsourced software development. It still offers traditional programming competitions, which it calls Single Round Matches (SRMs). But Codeforces is now the most popular platform for these types of online events.