Small list from me: Dungeon Crawl Stone Soup, Caves of Qud, Cataclysm Dark Days Ahead, ADOM, Reigns, SanctuaryRPG: Black Edition, King's Quest, Liberal Crime Squad. The old Elder Scrolls Arena and Daggerfall are also currently available for free. I see Daggerfall is playable with DosBox/Lutris, I assume Arena is as well.
I had hard time to properly get into Adventure Mode (gonna give it yet another shot once it releases in Steam Edition), I felt way too lost. It did however give me insane overpowered feel when I once decided to just become the BBEG and kill everybody in every town I stumbled upon, when it just let me do that using this incredibly detailed fighting system. Great stuff.
I didn't know about Approaching Infinity, and it definitely is something I'll enjoy a lot! Thanks!
What other game you can play as neckbeard otaku on a quest to become a real catgirl in the world infested with zombies and whatnot? God I love Cata