LÖVE, a free 2D game engine using the Lua programming language. Pure geek fun, from the adorable name and web site to the quality documentation to the use of a fun and clean language. Somehow the fact that I don't normally use Lua makes LÖVE more fun.
Obligatory Lua links:
lua.org, the official Lua site. You don't need to download the Lua interpreter to use LÖVE, but it's good to have around for command-line use and interactive experimentation. The site also has the 1st edition of Programming in Lua (now available in a much improved 2nd edition in print) and the Lua 5.1 reference manual.
Lua Nova, a blog about Lua that hasn't been updated since March 2008 but I'm linking to it because I'm about to crib his Lua installation instructions. :)
To install the original Lua interpreter from source using Mac OS X:
curl -O http://www.lua.org/ftp/lua-5.1.2.tar.gz
tar xzvf lua-5.1.2.tar.gz
sudo make install INSTALL_TOP=/usr/local
To install the Lua bundle for the TextMate text editor:
mkdir -p /Library/Application\ Support/TextMate/Bundles
cd /Library/Application\ Support/TextMate/Bundles
svn co http://macromates.com/svn/Bundles/trunk/Bundles/Lua.tmbundle
Then there's lua-mode for Emacs, and Lua support for other editors.
lua-users.org has a wiki, mailing lists and a chat room. luaforge.net hosts Lua projects.