The most polished I'm aware of off the top of my head is Spring (http://springrts.com/ http://springrts.com/), which started off as a Total Annihilation clone,

The most polished I'm aware of off the top of my head is Spring (http://springrts.com/ http://springrts.com/), which started off as a Total Annihilation clone, but grew into a pretty solid engine in its own right.