To start this category off, here's the most popular cross-platform IDE for Lua right now called Corona (windows and mac download):

http://www.anscamobile.com/corona/

Easily build iPhone/iPAD/Android etc apps from one code base! All the major phone apps already use Lua.

Lua is a non-MS open source tool embedded in .NET as a scripting engine and is truly cross-platform:
http://stackoverflow.com/questions/tagged/lua.net
http://ttuxen.wordpress.com/2009/11/03/embedding-lua-in-dotnet/
http://www.lua.inf.puc-rio.br/luanet/
http://www.gamedev.net/page/resources/_/reference/programming/sweet-snippets/using-lua-with-c-r2275

Lua would probably be considered a competitor to Razor as a scripting language. In addition to all the portable device programming it does, it supports multithreading and web development too.

I personally first saw LUA being used to write PlayON scripts embedded in .NET on Windows to access various video content from web sites on my TV. The first way to get this functionality from PlayON was to write a .NET plugin but it became MUCH easier to do after they intrduced doing the same with LUA scripts. The result: many more channels and much more content.