Released in candidate form a couple weeks back, the Spring RTS engine just released version 101. Spring is a real-time strategy game for Windows, Mac and Linux that is focused open source game engine.
Release 101 brings:
Lua Feature & Map shaders
It is now possible to set custom shaders for features and map drawing from lua.
Line of Sight refactor
Line of sight calculation performance was greatly improved allowing it to be smoothly updated every simulation frame.
The hard-coded transport behaviour was tweaked to allow the attachment of any unit to any other unit from lua.
Internal pr-downloader support
An essential step towards in-game lobby, it already allows rapid tags to be used in game dependencies and in start scripts.
Or you can view the new features in action in this video: