Programming

Coding challenges

• Project Euler
• Kaggle
• HackerRank
• Checkio
• JavaScript30
• TopCoder
• CodeWars
• CodeForces
• Advent of Code - 25 days of themed coding challenges, starting each year on Dec 1.
• LeetCode
• Project Lovelace - free scientific programming problems.
• Exercism.io
• PythonChallenge
• CoderByte
• CodeChef
• SPOJ
• Online Judge
• CodeSignal
• HackerEarth
• edabit
• Rosalind - Python ∩ Biology.
• practiceJS - some JavaScript coding exercises put into a web app so they can be done on a phone.
• CodinGame
• interviewing.io - free anonymous technical interview practice.

Developer portals

• GitHub developers
• Red Hat developer
• IBM developer
• Google developers
• Facebook for developers
• Oracle developer
• Discord developer portal
• Twitter developer
• Microsoft developer
• Amazon developer
• Apple developer

Languages

• C++
  • Standard C++
  • cplusplus.com - The C++ Resources Network.
  • cppreference.com
  • Microsoft docs
• Java
  • Java docs
• Elixir
  • Elixir docs
  • Elixir school - Premier destination for people looking to learn and master the Elixir programming language.
• Typescript
  • TypeScript documentation
• Kotlin
  • Kotlin docs
• Go
  • Go documentation
• R
  • R documentation
• Perl
  • Perl docs
• Haskell
  • Haskell documentation
• Rust
  • Rust Lang documentation
• D
  • D Lang documentation
• Clojure
  • Clojure docs
• Scala
  • Scala documentation
• Swift
  • Swift docs
  • Hacking with Swift
• Crystal
  • Crystal docs
• Nim
  • Nim docs
• PHP
  • PHP docs

APIs

• NASA APIs - NASA data, including imagery, eminently accessible to application developers.
• Reddit API
• Marvel developer portal - The Marvel Comics API.

Other resources

• Google TechDevGuide - Google’s Guide to Technical Development.
• Chess Programming Wiki
• Apollo Odyssey
• URF Academy by Riot Games
• Programming Idioms