Dead Man's Draw [Steam store page]
I really enjoyed the amount of variety in this single-player card game. There are some really unique card mechanics that keep it fresh and challenging.

Space Pirates and Zombies [Steam store page]
Slow but solid gameplay, though there are some major balance/difficulty issues - and to counteract those you need to grind quite a bit. The graphics are similar to a space RTS, though you have far less units than a normal RTS.