The latest Half-Life mod to resurface, repacked and polished on Steam is Halfquake Trilogy - defining 'masocore' well before I Wanna Be The Guy made it a genre. Halfquake is a trilogy of surreal and cruel puzzle-centric mods by Philipp "Muddasheep" Lehner and they're some of the most interesting maps produced for the original Half-Life. Featuring striking, abstract environments, genuinely clever puzzles, some very stupid ones and enough deathtraps to wear the ink off your quicksave key. They're worth playing, if just to test the water and your tolerance for pain.
Probably the best dissection of Halfquake comes from Noah Caldwell-Gervais and his deep dive into Half-Life's mods. Halfquake is three increasingly cruel adventures, and a challenge to rise to because it's there. While some comparisons can be drawn to Portal in its sarcastic all-seeing 'observer' character putting you through this hell, the tone is more cynical and - as Noah defines it - "dark, nihilistic and full of a kind of teenage ennui". You'll have to weather some slings and arrows to enjoy Halfquake, but there's a lot of satisfaction in overcoming its puzzles.
While the original Halfquake is - as the title suggests - halfway between Half-Life and Quake in its aesthetic, the latter two thirds of the mod are far more visually interesting. Stark monochrome environments, dark and abstract. Lots of dark rooms with only faint white lines to provide outlines, the incomprehensible void and a few trips into almost eye-searing white-space, just to mix things up. The slightly crackly old amateur voice acting doesn't hold up as well as the rest of Halfquake, but the visuals and music (a lengthy all-new soundtrack) hold up well.
Halfquake isn't the only classic Half-Life mod to see a Steam re-release. Probably the most notable is Sven Co-op, a completely standalone launch (no original game needed) including a co-op adaptation of the original campaign plus dozens more levels. Better still, it's consistently well populated - not bad for a nearly 20-year-old mod, eh?