Rebol is a modern interpreted language where code is data. It isn't object oriented, but has objects. It isn't a functional language but has first class functions. There are virtually no syntax rules or immutable keywords, making it ideal for developing domain-specific "dialects".
Rebol is a homoiconic language with heritage from lisp, forth, self, and logo. Language focus is on careful control of dependencies, and concern over the size of the interpreter and code...as well as achieving an English-like readability. The project's motto is "Rebel against software complexity".
On 12-Dec-2012, the unfinished ANSI-C sources of the Alpha release of Rebol3 was open-sourced under an Apache 2.0 License. Standalone "zero-install" binaries are available for x86 and ARM platforms--covering Linux/Windows/Mac/Android, 64-bit builds, hard-float compatibility and HaikuOS. Core builds are about half a megabyte, while GUI builds are around 1 megabyte (uncompressed).
(Note: Several forks of the code exist, with a community wiki summary at REBOL3 - what is the difference between the different branches?)
In addition to inheriting the "code is data" paradigm of Lisp, Rebol has a rich type system...natively recognizing patterns like URLs, dates, HTML tags, currency, and binary literals. Of note, Douglas Crockford (of JSON and JSLint) has credited Rebol as a source of inspiration for JSON, adding:
"Rebol's a more modern language, but with some very similar ideas to Lisp, in that it's all built upon a representation of data which is then executable as programs. But it's a much richer thing syntactically.
Rebol is a brilliant language, and it's a shame it's not more popular, because it deserves to be."
—Douglas Crockford, founder of JSON, 2009 [link]
A modern compiled variant of Rebol which draws in several new concepts is red, which is under heavy development.
An introduction to Rebol 2 has also been turned into 10 hours of YouTube videos. This introduction is being updated to cover Rebol 3.
Rebol: A Programmer's Guide on Lulu.com describes the features of Rebol 2
