USACO offers its own training program with a collection of practice problems sorted by increasing difficulty (you cannot access harder problems without solving the easier ones first though). Supports C, C++, PASCAL, Python, and Java.
Kattis is another practice site that also allows users to host their own competitions. It has an extremely large collection of problems and you can do them in any order. It also has a leaderboard and shows each user's rank based on problems solved.