Competitive programming

• Archived Problems - Project Euler : Problems Archives
• Art of Problem Solving : Is math class too easy for you? You've come to the right place!
• CodeChef : The only programming contests Web 2.0 platform
• Codefights : Test your coding skills
• Codeforces : Programming Competition,Programming Contest,Online Computer Programming
• Codewars : Rank up by completing code kata
• Codility : Verify and improve coding skills
• Codingame : Learn coding through games and challenges!
• Google Code Jam Practice and : past contest problems for practice
• HackerEarth - Programming challenges and Developer jobs
• HackerRank : Practice coding. Compete. Find jobs.
• PKU ACM ICPC Practice problems : Judge online for ACMACPC
• Sphere Online Judge (SPOJ) : Become a true programming master Learn how to code and build efficient algorithms
• Topcoder : Deliver Faster through Crowdsourcing
• UVa Online Judge : hundreds of problems supporting multiple languages.
• WakaTime : leaderboards of coding metrics collected via editor plugins