From now until the end of December you can head to the Shadow Complex site and download a copy of the game, though it’s worth noting this version comes attached to Epic’s own launcher—it’s neither wholly standalone (a la GOG.com) nor packaged through Steam. It sounds like you’ll need to wait until next year if you want to get the game through your usual channels.
The launcher situation’s a bit annoying, but I think a small a price to pay given Shadow Complex is a stellar Metroidvania game. And it’s free. And this marks the first time the game’s been available on the PC—previously it was an Xbox 360 exclusive. Not a bad way for Epic to convince you to install yet another game launcher.
As for the long-awaited sequel Developer ChAIR has an FAQ to deal with exactly that: “There is much more ChAIR would love to do in the Shadow Complex universe. Now that we hold the publishing rights, we’re hopeful that bringing the original game to multiple new platforms will increase the opportunity to do more Shadow Complex games in the future!”
I don’t think giving the game away free will help much on that front, but hey—I’m no economist.