is, according to its creators:
...a powerful, light-weight programming language designed for extending applications. Lua is also frequently used as a general-purpose, stand-alone language. Lua is free software.
I have found out about Lua a few hours ago, looking for a simple scripting language to embed in a game I am (trying) to write... It seems that everyone's beaten me to the idea of using Lua; Baldur's Gate, Grim Fandango, MDK2 and ClanLib already use it.
Anyway, I'm beginning to like it. It has got a clear syntax which isn't directly derived from C's (don't get me wrong, I love C, but everyone uses its syntax), and it has a lot of nice features. It provides features to add new features to the language, mostly using what it calls tag methods, which are methods issued on objects during the execution of the program, like assignments, comparisons et al.
Apparently, you can make Lua object-oriented with those, even though it isn't out-of-the-box.
Of course, it is very easily integrated into existing C programs, as you link to a library and add functions to it via a call-back interface.
It was made by TeCGraf, the Computer Graphics Technology Group of PUC-Rio (the Pontifical Catholic University of Rio de Janeiro in Brazil), a laboratory of the Department of Computer Science.
And, as they say... Lua means Moon in Portuguese
More info at: http://www.tecgraf.puc-rio.br/lua/