1.Spoj.com
SPOJ (Sphere Online Judge) is an online judge system with over 315,000 registered users and over 20000 problems. The solution to problems can be submitted in over 60 languages including C, C++, Java, Python, C#, Go, Haskell, Ocaml, and F#. SPOJ has a rapidly growing problem set/tasks available for practice 24 hours/day, including many original tasks prepared by the community of expert problem setters associated with the project.