Dokimon Quest, one of the more interesting and nostalgically gorgeous Pokemon-like games around, is totally free to keep on Steam. But you'll have to act fast if you want to join the 1 million people who have already claimed the pixel art monster tamer.
Developer Yanako RPGs made its throwback monster-collectathon available to Steam goers for the small fee of nothing just yesterday. The offer runs until tomorrow, so there's not long left if you want a free shot of GBC-era nostalgia heavily inspired by Evangelion. If that doesn't sweeten the pot, I don't know what will.