Code practice and challenges

I am going to try to compile a list of code practice, challenge, and ongoing contest sites that are helpful for learning and refining programming knowledge. It seems like there have been a bunch of these popping up lately, and having a list of new and old might be helpful.

• hacker rank focusing on algorithms and AI
• code game
• Project Euler focuses on math
• Rosalind is similar to project Euler except with a focus on bioinformatics.
• Code chef focuses on training for programming contests and short challenges