From the Depths is a vehicle designing game with a focus on naval combat. It has a fairly steep learning curve but offers tons of custimization which allows for so many interesting designs including ships, helicopters, planes, spacecraft, submarines, hovering fortresses, and structures. It has a large number of in-game tutorials to walk you through the learning process and several single player campaigns. There are hundreds of very impressive ships built into the game for the AI forces in single player. There is also multiplayer but they are mostly private games.
The developer has been very active over the past year, regularly releasing both new content and bug fixing updates.
From the Depths is one of my favorite games. I bought It for 20.00 USD but right now it's on sale for 4.00 USD, 80% off!
I strongly reccommend everyone on Zero-K to consider buying this game while it is still on sale (ONLY 43 hours left from the time of posting this) because if you can conquer the somewhat intimidating learning curve, it will provide hundreds of hours of fun.
Simple trailer showing some designs
Longer trailer showing designs and other game features