It's not free on Steam, but you can download Mindustry for free from the developer. It's basically an open source version of Facterio, and has a ton of options. You lose out on the one-click installation of mods and schematics from Steam, but it is pretty easy to download and install those yourself.

There are quite a few classics like, Path of Exile, Runescape, Dota, Spiral Knight, Unturned Brawlhalla, Fishing Planet

You should try the Steam Search - with some filters it's quite powerful

e.g. https://store.steampowered.com/search?sort_by=Reviews_DESC&as-reviews-score=80-&as-reviews-count=5000-&maxprice=free&category1=998&os=mac&ndl=1

• OS: Mac
  
• only Free
  
• type games
  
• min review score 90%
  
• rated by 5.000 people (lower to get less popular titles)
  
• sort by review score

Epic Games has free game giveaways every week and many times they have Mac ports too. For example I got Dead Island 2, Dredge, Darkwood, Humankind, The Long Dark, Night in the Woods, Obduction, Pathfinder: Kingmaker — Enhanced Plus Edition, Road Redemption, Survivng Mars, Infinifactory, Tannenberg and Verdun for free there.

This thread was quite old before the recent post, so we're locking it to prevent confusion.