Leet Code is a great site with some really interesting problems.
Code Wars is another site to practice coding. I like the interface!
Github code is a must for aspiring programmers.
CodingBat.com I use Codingbat in my Java class very extensively.
Hackerrank profile Hackerrank is a great community with excellent problems.
Khanacademy profile I filled in so many gaps in my knowledge of math on this site.
Stackoverflow profile I often use Stackoverflow to help me answer programming questions.
Freecodecamp profile In order to learn about web programming, I’ve used Free Code Camp.
Codecademy projects This site is geared mostly towards beginners, I would say.
Vocabulary.com profile Students looking to study for their GREs would find this site useful.