There are many problem archives online, each with its own characteristics. Project Euler has math problems where answers are submitted via plaintext, TopCoder has its own flavor of problems with an ancient Java applet interface (although it has a new, still very primitive, web version in beta).
Compared to other problem archives, the Kattis Problem Archive has a very high ratio of quality/quantity. Also, the average problem statement on Kattis is more interesting than the average problem statement on TopCoder/Codeforces (e.g. "Fox Ciel just got this sequence of integers as a birthday present from his grandma, ...".)