Thanks to the launch of mod support via the SDK, you can run your old Quake and Half-Life maps in retro shooter Dusk. And it looks really good.
The Dusk SDK allows for the usual texture replacement and custom sound mods, which predictably results in a lot of stuff like, for instance, "Tim Allen Textures" or a Winnie the Pooh replacement for Leatherneck. But it's the ability to run external maps that I get a real kick out of. It takes a little effort—you'll have to extract the maps from Quake, copy the Quake palette.imp file to the proper location, and do some other stuff that you can read about here—but the net result is, quite literally, Quake in Dusk, albeit a Quake populated with new, thematically-mismatched enemies.
The SDK also enables the use of Half-Life maps, if that's more your thing, and more importantly user-made maps for either game should also work, with some limitations: The FAQ notes that compatibility with user-made maps isn't guaranteed, and large Quake maps "can technically be loaded but currently experience significant issues."
Dusk mods are currently available on Mod.io, but publisher New Blood Interactive said that Steam Workshop support for Dusk and a Mod DB presence are "coming soon." The full lowdown on Dusk mod support and the SDK are available on Steam.