Lua

Lua is a very nice and easy to learn scripting language specifically designed to be embedable and able to represent complex data.

It features:

• Clean syntax
• Simple yet extremely powerful and extensible datastructure, the tables, that can be used as hashtables, vectors, arrays, lists, namespaces, objects, ...
• Easily embedable into existing projects
• Lightweight
• Very fast
• Can be compiled to bytecode(actaully it always is, but the bytecode can be dumped to a file for later reuse)
• Many other nifty things ;)

Lua is heavily used by some roguelikes, like ToME, where a good part of the engine is in Lua and the game itself will be completly in Lua in the next version, and the H-World engine.

See also Lua homepage.