Io is a small, prototype-based programming language. The ideas in Io are mostly inspired by Smalltalk (all values are objects), Self (prototype-based), NewtonScript (differential inheritance), Act1 (actors and futures for concurrency), LISP (code is a runtime inspectable/modifiable tree) and Lua small, embeddable).
Besides running in all the usual platforms, and some not so usual ones like Symbian and Syllable, it offers an interesting set of libraries including sockets, databases, OpenGL, some crypto APIs or, notably, and Objective-C bridge.
If you’re interested in prototype-based languages, and want to try something simpler than Slate or newer than Self, Io looks like an option worth considering. Besides, applications like this one seem to point to a relatively mature language.
On a loosely related note, schemers interested in prototypes (like myself) may find Jorgen Schäfer’s Prometheus an excellent way to get acquainted with this fascinating subject, and maybe spend a couple of fun evenings implementing selfish patterns (as explained in the article by Brian Foote that gives name to this post) in Scheme.