The new IndieRoyale includes Serious Sam 3: BFE, along with other 5 (smaller) indie titles. I only tried Lunnye Devitsy from that bunch and it's a good small and simple game (only 2/4 hours of gameplay though), but I never played any others (except Wake, from the same author, only for a few minutes).
Note that not all games are on Steam, as some are on Desura and/or DRM-free.
Anyway check yourself
http://www.indieroyale.com/