DMOJ is described as 'Practice problem solving and compete against others on the DMOJ, a modern contest platform and archive of programming problems'. There are five alternatives to DMOJ for Online / Web-based, Self-Hosted solutions and SaaS. The best alternative is HackerRank. It's not free, so if you're looking for a free alternative, you could try Spoj or Topcoder. Other great apps like DMOJ are Codechef (Free) and CodeSubmit (Paid).
Sphere Online Judge created for the coders' community to train, to learn and to make progress. With thousands of programming problems and an international group of users Spoj is one of the best programmers' communities and online judges in the world.
CodeChef is a not-for-profit educational initiative by Directi, an Indian software company. It is a global programming community that fosters learning and friendly competition, built on top of the world’s largest competitive programming platform.