Spring engine
However you need some sort of interface to play the thing, so I advise spring lobby. It works with every game pack out there IIRC.
Spring Lobby
And then there's zero-K (formely called complete annihilation) which IMO is the best game for spring engine available. Balanced annihlation has a lot of users, but I find zero-k much more fun since it has a flat tech tree. All units are useful.
Zero-K
Here's a youtube video and some screenies of zero-K in action:













