The first version of Space Usurper was made for the (currently abandoned) Approval collection of minigames:
The second version was released in Facepunch Prototypes:
The main new feature was that enemies were designed and shared as a PNG image. Though this version got a lot of positive feedback not many people used the PNG feature and it has not made the jump to the third (and hopefully final) version.
If you want to follow this project you can sign up to the mailing list.
We'll only update you about this project, we won't spam you about other stuff or sell your email address.