The Half-Life 2 Spherical Nightmares mod is good and you should play it
Some mods want to re-invent the game they're based on, but when that game is Half-Life 2, there's no need. In mods like Minerva (highly recommended, if you haven't played it), the creators treat Half-Life 2's weapons, enemies and environments as building blocks, playfully rearranging them into new scenarios to hone the craft of level design. With no fresh art or spectacle to fall back on, it's up to the pacing of each environment to keep the player fascinated.
New singleplayer mod, Spherical Nightmares does a rather good job of that. You start in jail, interred (ineffectively) by the G-Man, who reprises his role as Creepy Distant Voyeur Dude from the Half-Life games. Your first steps are cautious and exploratory, but as you clamber out of the dark bowels of the facility you'll stumble into escalating firefights that pay homage to Half-Life 2 and - in a challenging train ride section - Half Life 1 as well.
There's a haunted house vibe to the dimly lit starting corridors as well. It never stoops to FEAR level scare-craft, but you'll walk into black and white flashbacks showing a time when the facility was full of people yet to be munched on by an army of invading headcrabs. It's clear from the disturbed inmates and foggy Blair Witch visuals that ALL WAS NOT WELL. Look at this man walking pensively away from a game of chess. Who is winning? Why is he leaving? Probably because a strange man in a yellow suit with a crowbar is photographing him through the glass. Sorry, man.
There is shooting. It is a lot like the shooting in Half-Life 2. This is no bad thing. The first proper fight is a slightly unexpected change of gear, but it moves from a vantage point with a heavy machine gun down into a courtyard with a LOT of combine soldiers in it. Here I rediscovered the pleasure of the right-click twin barrel shot on the Half-Life 2 shotgun, and there's a sneaky Ravenholm razor in one corner for Gravity Gun aficionados. It's easy to forget how challenging the Combine can be in numbers. They're still not terribly good at understanding cover, but the way they split, flank and use grenades is unnervingly effective.
The courtyard fight contracts into a vertical staircase fight (more good news for shotgun fans), which marks a slight shift in pace that feels like a studied attempt to recreate the gentle rhythm of Valve's levels. Half-Life partly controls pace with the expansion and contraction of space, ushering players through its sections with a subtle peristalsis motion that's emulated here. The brief sewer section teases daylight a few times before you emerge into the sunlight, another thoughtful touch.
It looks good, too. Spherical Nightmares embraces the extra frills of the Episode 2 version of Source, and is lit sparingly. The engine looks increasingly dated, but the decision to use Valve's cohesive assets keeps things clean and unified and there's a lot of variety in the two hours or so it takes to complete. Don't expect much in the way of story resolution, however. Perhaps we'll learn more about the sphere and its innards in a follow up, but that won't be any time soon. Spherical Nightmares took five years to make.
Fancy giving it a shot? You can download the mod now from ModDB. Extract it to your Steam->steamapps->sourcemods folder, restart Steam and it'll pop up in your library. you'll need Half Life 2: Episode 2 for it to work. See if you can find this guy on your travels.