Mac OS X? But that's only for the NeXT inheritance.
Last I heard, he develops on a Mac and iD switched over to Mac once the x86 machines came out. (IIRC IdTech5 was first shown on the mac as well). Then of course there is also Ryan Gordon who uses Mac as well.
idtech3 and 4 (Q3A and Doom3) was also first shown off on Macs too iirc, and a Mac q3test client came out before later Windows q3test clients also (though Linux also had a client before Win too so maybe that last one doesn't count)
There's of course old .plans of Carmack touting how OS X is going to be the the greatest thing ever since NeXT was absorbed.
I'm going to have to beg to differ on two points here:
1. FPS games often have scripted cinematics, collectable equipment, and other elements that have significant cross-over with the RPG genre (however inventory management and a lot of UI work must be added).
They also have ample amounts of AI controlled creatures, which brings them close to RTS (however map-wide routing and sector-based strategic AI must be added).
You can even look at the selection of Quake mods to see a handful of RTS or RPG games that didn't even modify the engine code at all (some of them predate the release of the source)...
All that said, there are fundamental design differences one tends to see in RTS games, RPG games, and indoor FPS games, chiefly to do with outdoor landscape handling (RTS, RPG) and stream loading (RPG).
2. UnrealEngine is clearly designed for indoor/outdoor FPS (while some other engines are solely indoor FPS focused), and requires significant tweaking for each title, which makes me question the "not FPS" claim.
It's also not very friendly to modders in terms of flexibility (as I learned the hard way when doing contract work on a game prototype in the form of a UT3 mod), proving difficult to turn into a non-FPS game without surgery on the engine code, but I assume you were speaking only of licensees.
I don't think so. Collectable equipment doesn't make your FPS an RPG. This is a huge misbelief that went into mass production. Just because you can put on a couple of suits with better protection doesn't make an RPG. If the item is just "better" than a previous item it's not RPG since RPG involves "choice". Go see Deus-Ex to understand when an FPS has RPG elements. Nearly all FPS games today boasting with RPG elements have none at all.
Also the AI of FPS games is in general lame. I've yet to see an FPS where the AI is not so stupid it hurts. Even FPS games which boasted with superior AI ( FC for example but many others there are too ) had shit AI if played by somebody who knows his trade. RTS games are though not necessarily better in AI but FPS is usually not what I look for to find good AI.
And because mods don't change routines to implement an RTS for example doesn't make an FPS equal to an RTS. For a real RTS you need to change the AI since FPS AI is _not_ suitable for an RTS as simple as it gets. Going cheap during modding doesn't change that fact.
What goes for UEngine it's rather flexible and can be bend into all kinds of direction. Nobody said it's easy to do but in contrary to other engines it _can_ be bend and that's the important point.