Just finished Black Mesa, with the additional mods to extend We've Got Hostiles, On A Rail and Surface Tension.
In case you don't know,
Black Mesa is a complete remaking of the first Half-Life, in the Source engine, that's been put together by a modding team over the last nine years. They don't use any existing assets, except for a handful of things that're in Source anyway (headcrab models, some HEV suit noises), and the game isn't a direct port, more a re-imagining, within the context of modern technology and gameplay tropes. Still follows the same story, although there's no Xen currently as the final chapters are being developed for Black Mesa's 'retail' release on Steam.
It's very slick, there are barely any stability issues, and it's hard to believe it was all developed pro-bono outside the studio system. It feels not only like a professional game, but an extremely
good and well-made one. The levels are jam-packed with detail, the combat's exciting, the level design is brilliant, the lighting in the maps is
spectacular.
The funny thing is that Source isn't some cutting-edge technological showpiece anymore, and is almost as old-hat as HL1's Goldsource was at the time Black Mesa began development. But the levels are so well lit, textured and decorated, that the game looks utterly gorgeous nonetheless. Sensational piece of work, and the best game I've played since Portal 2.