While there's going to be a lot of obvious comparisons to Factorio due to the production chains and mining, Mindustry still manages to come off quite unique despite the similarities. Mindustry has a much bigger focus on the action and it has a different more simplistic art style to it.
It's free and open source, with the code up on GitHub but now that it's on Steam they can support the development even further with a much bigger audience. Additionally the Steam version does have some extras like Achievements, Steam Cloud, easier multiplayer and Steam Workshop support.
It has multiple games modes including Survival which is a tower-defense styled mode to survive as long as possible, an Attack mode to build factories and units to destroy enemy cores while defending your own, a PvP mode against other players and a Sandbox mode to do whatever you want with no enemies unless you want them.