• https://github.com/vhf/free-programming-books: huge list of free books and algorithmic problem sets

• https://github.com/OpenGenus/cosmos similar to this with more algorithms, but where are the tests?

• http://algs4.cs.princeton.edu/home/, which has lots GPL Java source. This kind soul has put the source up on GitHub: https://github.com/aistrate/AlgorithmsSedgewick

• http://www3.cs.stonybrook.edu/~algorith/. Links to tons of open source algorithm implementations that solve many problems. Each algorithm has a rating, and algorithms are all classified.

• http://www.geeksforgeeks.org/fundamentals-of-algorithms

• http://en.wikibooks.org/wiki/Algorithm_Implementation

• Skiena - Algorithm Design Manual 2ed
• Cormen - Introduction do Algorithms 2ed

My favorites are:

• TopCoder. No solutions. 6M registered users. Money prizes. Some company proposed problems have Non Disclosure Agreements. Timed submission contests.
• HackerRank. No solutions, 3M Round A. Timed submission contests.
• Kaggle. Data science focused. No solutions. Some problems have money prizes.
• Sphere Online Judge (SPOJ). No solutions. 200K users, 10000 problems.
• Project Euler. 350K users, only ~450 problems. No solutions. Since 2001. Probably one of the oldest around, but did not evolve much.
• UVa. 100K registered users. No solutions. Slow website.
• CareerCup: Interview questions.

Yearly contests:

• ACM International Collegiate Programming Contest. Wiki. No solutions. Eligibility: less than five years of university education before the contest. Started in 1977. World final problems.
• Google Code Jam. Only a few solutions.
• ICFP. One problem per year. Since 1998.