1. Buy Thief Gold from GoG ($9.99) [1].
2. Install this [2] community patch to downgrade it to The Dark Project.
3. Enjoy!
Always look for any old games on GoG first. If you can't find a game there, you'll definitely find it on either retro gaming forums or private torrent trackers.
Sadly AFAIK he's still doing that anonymously through a french forum or something, probably because legally he shouldn't have the code. Despite that all modern re-releases of the games that used the Dark Engine (like those on Steam and GOG) are based on NewDark instead of the original engine.
Though none of these are current day, they are not old games.
WoW at least has had a recent update (including a basically total graphics engine rewrite last year, switching to a deferred pipeline, etc) and still runs well - I'd call it more recent, but maybe that's just me.
The only issue is that on exit to desktop the game hangs and needs to be manually killed.
The new DX11-only re-release doesn't work though.