O is a esoteric programming language used for CodeGolf. It was inspired by languages like GolfScript, Pyth, K, and ><>, but it has grown into its own language with many cool features. The current interpreter is written in C. You can find the old Java interpreter on the java-interpreter branch.
All documentation for the language is on ReadTheDocs. You can try the language out online at Heroku.
Here are some examples of O.
"Hello, World!
H,;]*
i
0J;1Kpj{K:VJ+:KpV:J;}d