When Black Mesa launched in early access in 2015, the Valve-sanctioned fan remake of Half-Life was missing the final chapters set in the alien plane of Xen. Because this part of Half-Life was disliked by many (pssh I like it), the devs wanted to rework it rather than just remake it - a process which ended up taking another five years. Now they've released a playable 'Xen Museum' showing what they were up to all that time, with loads of different versions of Xen's maps from across the course of development. What a delightful idea!