Completely free site that teaches the most common languages in use today either in your browser or using mobile apps that are capable of working offline.
Ranked in these QuestionsQuestion Ranking
Pro Completely free
Pro Offline learning mode for mobile apps
Allows learning while disconnected from the net.
Pro Easy for beginners
Anyone can get started with this.
Pro Excellent mobile apps available
XP, levels, badges, certificates, etc.
Pro Share and modify others' projects
Pro Browser-based code playgrounds
No software installation needed, just a modern web browser. On mobile devices though the mobile apps are highly recommended.
Con Only the most commonly used languages are covered.
Con Limited usefulness for intermediate or experienced programmers
No advanced coding challenges. Look for those on other sites like hackerrank.