That's a tough one because ROTH is really unique.

Roughly sorted from most similar to least similar:
Clive Barker's Undying offers a similarly great horror atmosphere, as well as wonderful worlds and story, but without adventure elements and talkative sidekick. Unfortunately not available on GOG
The Penumbra Games and Amnesia (available on Steam) do first person horror adventure really well. but they don't reach the scope of ROTH with its fantastic worlds.
Gremlin's Normality is the only other 3D-adventure I know apart from realMYST (both available on GOG), but they lack the horror elements and apocalyptic story. Still, realMYST offers some fantastic worlds and a subtle but good story.
The seventh guest (not on GOG), Scratches and Alone in the Dark (on GOG) have a similar locked/haunted house motif, but now I'm really reaching for straws ;-)

I've played Penumbra Series and Amnesia (I'm a fan of Frictional Games).
Bought Scratches on steam some time ago but haven't installed it yet.
I'll try Clive Barker's Undying.

Thanks for the reply.

I will totally, absolutely second Undying. Great game.

Sadly, what makes Realms... Realms... (for me) is the interface, inventory, and how interactive everything is. Its combination is so unique.

A game developed by many of the same people, and similar at least in occult theme is Soulbringer. It came out around 2000. I read somewhere, I forget, that it was a spiritual successor, so take that for what you will.

en.wikipedia.org/wiki/Soulbringer

I'm very happy to be playing this again, and have for years looked for games that can hold a candle (Tallow, I believe) to the mighty RotH. I agree with Undying; for a similar atmosphere, I found Eternal Darkness on Gamecube not far off.

Eternal Darkness is a good one. Really similar structurally with a haunted house as hub and various fantastic worlds that are visited from there.
I just remembered the "Wheel of Time" FPS by Legend Entertainment. Since ROTH IMHO lifts a lot from Wheel of Time (see here) there are a lot of similarities and the level design is really varied and surreal.
Coincidentally, since both Undying and Wheel of Time are using the Unreal engine I also want to recommend the original Unreal (Unreal Gold on GOG) for its fantastic level designs and the world that is created through it.

Just started playing this yesterday (bought a boxed copy two years ago).

Thing it immediately reminded me of was the first System Shock game, and by that token the Ultima Underworld games. Arx Fatalis is in the same ballpark, I guess, but the UU games and Arx weren't really /scary./

SS1 was, though. Regrettably, it's not on gog.

Call of Cthulhu: Dark Corners of the Earth also pops up.

Undying is an action game, more than an adventure with action as ROTH, but the atmosphere and the plot, everything on it, is awesome.

Ultima Underworld 2 is almost the same game, in many ways. Some folks called ROTH "Ultima Underworld 3" in fact. It is available here, on GOG. Despite it's age, it is still well worth playing, a Classic.
As far as creepy feel and cool plot, the original System Shock is great. If you search for "System Shock Portable" you can download a full version of the game that is playable from a usb stick, or your hard drive, with resolution up to, erm, can't recall right now, but it is very high. Great work, whoever packaged it.

Yet no one mentions BLOOD...???? It's a sad day indeed on GOG.

I kinda think the early Resi Evils and Silent Hills are sort of similar - horror with shooting and puzzles!

Peopel have also mentioned the Systemshock series without Bioshock...

Hitherto ignorant of its existence, I've only just now got around to looking into the Wheel of Time game you suggested, thanks to Fever who bumped this thread up. Judging from the first hour I've spent familiarizing myself with the interface, I must say it's of some peculiarity that I haven't happened upon this game before. I can already attest to an unconventional beauty, partially recalling the singularly riveting atmosphere which has made me become so fond of Realms of the Haunting. So kudos to you for that most interesting recommendation! I'm quite excited to explore the other levels, such as the Ways that I've read about in another thread regarding WoT's Windows 7 compatibility. I know the Tower in RotH has inspired some comparison to that landscape. Other than that, the impression I've gotten so far is that the considerable adventure aspect of RotH (in terms of exploration, evaluating documents and items, etc) doesn't seem to be quite as prevalent in this game, though. The gameplay is more concerned with fast-paced action I suppose, a bit reminiscent of Painkiller, but the intricate level design is enough to keep me hooked for now, haha.

Ressurecting an old thread because GOG just released a game that some have been comparing to Realms of the Haunting: Albedo: Eyes from Outer Space.

People looking for somewhat similar experiences to RotH should give this one a try, it's not as serious as RotH -- it's intended as a sci-fi pulp video game inspired by early 20th century pulp novels and 60s sci-fi movies -- but it's the closest mix of adventure with action in first-person perspective to Realms of the Haunting, as far as I know.

Realms of the Haunting is still the better game out of the two, but, then again, it was always the better one out of all the suggestions people have been making in this thread (as much as I love most of them, mind you, they're either not the same kind of game RotH is or they're just not as good).

If you're remotely interested, be sure to give it a try. It doesn't have the charm of Realms' early video game FMV cutscenes, but there's a certain pulp-ey, campy vibe that Albedo gives off that feels a lot like our beloved RotH. Plus: one man did that game entirely on his own.

Thanks for the link. This one totally went under my radar, which is really strange, since I shop at GOG regularly.

Anyway, yeah, totally different subject matter and vibe, and yet from watching the video, I can see glimpses of a similar art direction, and possibly other, more subtle similarities, such as a vivid imagination being used with regard to settings. Also, apparently there is WASD for movement, and mouse for other interactions, so that is similar as well.

Difficult to glean anything from that video though, so I could be in fact wrong, but I am intrigued, and have added it to my wishlist for now.