^^ WTF, it didn't show up when I searched for it! I tried both Moby's internal engine and using Google to search the site. Sorry for the misdirection in any case. (Edit: just noticed Moby have it listed under "Windows" although that doesn't explain why search failed.)
Mad Space is one of those ones that I like a ton even though I've never played it all the way through & don't even think it's very good. The gameplay is honestly kinda crap (and SO VERY UGLY) but it was super innovative back in the day. See my comments elsewhere about preferring games that were unafraid to try things out of left field. My next clue was going to be "non-euclidian geometry" which it used all over the place (hence the name!), AFAIK it was the first FPS game to do so in its original release. (There were already some community-made maps for Descent 1 & 2 that played with it in a similar way.) If you liked Antichamber or Portal, well, this one did it first.
It also supports the VFX-1 (albeit with the left & right eyes reversed?? I haven't been able to get that set up right yet!), 3D shutter glasses, and voice commands via speech recognition.
Fun fact: it's also on Steam so you can still buy it to this day. 😜
The floor's yours Mr Flibble!
twitch.tv/oldskooljay - playing the obscure, forgotten & weird - most Tuesdays & Thursdays @ 6:30 PM PDT. Bonus streams elsewhen!