Yeah, I realize that hardly anyone plays the game Half Life anymore for any reason but nostalgia.  Even with mods, though, what engine are you using?  Who's net code? And, if it's a less expansive mod, who's textures and models?  The point is that wether it be a mod to tweak the original gameply or a total-conversion, you're still using the original game to play it, therefore developers could still collect their $18 a year.