Io is a prototype-based programming language inspired by Smalltalk (all values are objects, all messages are dynamic), Self (prototype-based), NewtonScript (differential inheritance), Act1 (actors and futures for concurrency), LISP (code is a runtime inspectable/modifiable tree) and Lua (small, embeddable).
It features BSD license, small vm (~10K semicolons), multi-state (multiple VMs in same process), incremental garbage collector, weak links, actor-based concurrency, coroutines, 64bit clean C99 implementation, embeddable, exceptions, unicode.
Above from [L1 ]
Remarkable features of Io are its minimal size and openness to using external code resources. Io is executed by a small, portable virtual machine. [L2 ]
Like Tcl, TRAC, Rebol, Prolog and Lisp, Io is homoiconic [L3 ] More info at [L4 ], [L5 ] and [L6 ] for more info on these types of languages.