Zero-K is a GNU-GPL open source RTS game, aims to be dynamic, action-packed and hassle-free.
The design philosophy is quite different to the Blizzard RTS games. It doesn't have factions, but there are 100+ unit types. There is no unlocking requirement for techs. The unit balance is largely regulated by in-game physics instead of less intuitive mechanics like damage multiplier. The UI is very powerful, perhaps the most powerful among all traditional real time strategy games.
It was originally started early 2007 as a Total Annihilation mod called Complete Annihilation on Spring engine, with a genealogy that could be traced back to Uberhack, a community balance mod that was popular around 2000. The copyrighted contents was removed later.
A former Zero-K developer, also a developer of the indie game Istrolid, explains the gameplay: