Half-Life: Blue Shift is now available on Steam for people who own the original Half-Life, or one of the two Half-Life 2 Steam "let me download all the games cos, like, I gave y'all lotsa money" packs. [The Silver and Gold packages. - Ed]
It's not a remake - it's just a handy way to get hold of one of the original game's official expansion packs. Opposing Force is already available, as you may know, along with favoured mods like Day of Defeat (which has just been patched slightly, by the way), Deathmatch Classic, Ricochet, Team Fortress Classic and Counter-Strike.
For those with short memories (or perhaps with memories now neatly reorganised to provide prime wistful-recollection space to the greater moments in Half-Life 2), Blue Shift saw players take control of security guard Barney Calhoun as the nastiness at Black Mesa started to unfold. It was an entertaining effort, although it didn't last very long - in fact, you could probably play through it in an evening.
Probably should, too, since the chances are it's free to you. Reload Steam and you should see it - right-click and install and you're away.