Neon Struct [official site] is one of the 25 best stealth games on PC, says our Adam, and who am I to disagree with him? Him with his gentle voice and wry smile. And, uh, apparently his sneaking skills good enough to write that previous sentence when I popped into the kitchen for a cuppa. Well, no matter. Point is: Neon Struct was already grand, and now it's practically endless, as it has a level editor with Steam Workshop support. Minor Key Games have also dropped a free expansion. How splendid!
You too can make your own Neon Struct levels, though you might want to have a read of the documentation first. From there you can upload them to Neon Struct's new Steam Workshop or, I believe, release them as standalone downloads. I myself will award a special No-Prize to the first level infiltrating a dastardly cyberpunk girl gang.
Head on over to the Workshop now and you'll mostly find The Dulce Archives, that new free expansion. It's a prequel to Neon Struct's main story, set 18 years earlier in the distant past of 1997, when something fishy is going down in New Mexico.
Here, this trailer shows off bits of the level editor and The Dulce Archives: