Half-Life, UT99 & Classic Doom, my personal gaming trilogy, three of the best FPS ever made. That's why I'm regularly searching for mods/maps that expand the pleasure of playing these games.
Last August 10th "Half-Life: Echoes" (SP mod for Half-Life) got released and let me tell you, it's a blast. A mod that feels and looks more like an official expansion pack developed by a single guy in 4 years. If you guys have a copy of HL (Steam version), then you must play it, it will blow your minds (attention to detail in each level is just impressive, let alone the astonishing coding, the use of A.I. and a storyline that actually fits Valve standards).
I hadn't been this impressed about a mod since Blak Mesa (the mod, not the retail-delayed game), and I think Echoes turns out to be much better than BM in many ways (personal opinion) e.g., Echoes is a full game whereas BM mod was never completed, and retail BM has been an early access game for more than 3 years.
Here's the official mod page at ModDB: Half-Life: Echoes