As you may have noticed by now, Perpetuum is now available for free on Steam.

I wish we'd have some grandiose reason as to why we did it, and why now, but really, it was just the point where some of the paperwork overhead didn't seem worth it after a while, and after we've recouped some of the costs of the initial server shutdown, it no longer seemed fair to ask money for a multiplayer game that's essentially entirely operated by the community itself - while we could justify it by still occasionally patching the game when really major bugs appeared, this has become so uncommon that the justification has become thinner and thinner.