Beyond All Reason is quietly building up to be one of the best real-time strategy (RTS) games around. It's free, open source and it has a fancy new trailer.
The Recoil engine is interesting to me. It's not something I'd seen before in the open source ecosystem (sometimes I live under a rock). I suspect the underlying mechanisms (although written for RTS games in the Total Annihilation family) might permit implementing something like a Paradox game with some tweaking.