Yep, Valve's unfortgettable opening salvo officially gets a decade under its belt on this very day. This terrifies me in ways I can't quite vocalise.
Said terror is not simply because it makes me feel so very aged, but also because it's been so long and yet still most FPSes seem entirely ignorant of what HL did with setpieces, incidental detail, gaming-specific narrative style, pacing and world-building. The straight line to most of today's shooters that started with Doom seems to almost bypass Half-Life. Fortunately, there is one huge, positive side-effect which does ensure HL's legacy is unassailable: modding.
Names like Counter-Strike, Day of Defeat, Team Fortress Classic, Natural Selection, The Ship and Sven Co-Op trip off the tongue almost as quickly as Half-Life itself. The GldSrc engine might look ten years old by now, but it's still one the driving forces of community and customisability that defines modern PC gaming and the freedom thereof.
In honour of the occasion, Moddb's mustered a round up of Half-Life's notable mods, both commerical (the list) and non-commercial (the video). It's all too easy to fall into thinking HL modding's just about Counter-Strike clones and grey weapon renders, but even this collection of the more obvious names makes it clear quite how landmark a modding platform this game was and is.
Happy birthday Half-Life, you wonderful old man, you.