Overgrowth has been playable in alpha state since 2008, and five years laters and more than 200 updates along Wolfire still haven't released the finished product.
It seems that fact won't change for a while, but the game is now available on Steam Early Access, meaning you'll be automatically updated as the game is being worked on.
Currently the game is available for £ 19.08 with a 17% discount.