Highly evolved engine based on id Software technology, available under dual license (GPL, proprietary licensing for commercial use available).
Engine of choice if you still dabble in quake one.
What can it do?
Ton of visual effects & rendering magic.
It CAN load Quake 3 maps! Yes, quake 3. All shaders arent supported, but this is one heck of an achievement. Also loads half-life maps.
Super customize quake-c code - create console vars on the fly, detect map information like texture names - there is a massive extension list in dpextensions.qc.
Colored lighting for your maps.
This list goes on.
Open source, so you could potentially customize it further. Though so far in using it for my mods for the last 10 years or so, I havent found that to be necessary. It loads my biggest maps, and heaviest models, which just kill other engines. Before darkplaces I was customizing glquake and tenebrae. I can tell you its hard enough developing a big mod without having to customize the engine driving it on the side.
One time I needed something done and my modified dp engine code wasnt working right, so I contacted the developer, LordHavoc - he responded pretty quickly and told me how the engine already did what I wanted and how to access it. Awesome!
This engine single handedly saved me a ton of work porting my quake mods to something like the source engine or unreal sdk.
If you still quake one, to play mods, deathmatch online or watch demos, I recommend darkplaces.