Lua
Lua is a splendid scripting language. It's simple, powerful, fast, and easy to extend and embed. Since it's written in pure ANSI C, it's highly portable; the source also compiles as C++. It's small: a typical interpreter executable dynamically linked to the C library is under 150kb. See Lua's home page for more information. Lua is open source. Versions for many operating systems are available from the Lua Users wiki.
I use Lua a lot, and have written or maintain the following libraries and other bits and pieces:
- stdlib, a collection of Lua modules that aims to become a standard repository
- lrexlib, an interface to PCRE and POSIX regexps
- bitlib, a tiny library for bitwise operations
- clue, a minimal Lua to C binding
- Patches to prevent automatic conversion between strings and numbers, make print work with strings containing NULs, and improve signal handling in the lua standalone on POSIX systems; currently up-to-date for Lua 5.1.2
- EPOC port of Lua with OPL integration (currently at version 5.1.3)