The bad news is that Blue Shift, the actual game, doesn't amount to much on its own terms. For good measure, Blue Shift also includes the Half-Life High Definition pack, which refurbishes some of the graphics throughout Half-Life and makes the entire game look a little newer. In North America, Blue Shift also comes bundled with Opposing Force-and you don't need the original Half-Life to play either of these. Thus, plans for the release of Half-Life for the Dreamcast changed, which is why Blue Shift has now been made available for the PC. Blue Shift itself was originally developed as supplemental content for Half-Life for the Sega Dreamcast, but unfortunately, that console was discontinued earlier this year. Blue Shift was developed by Gearbox Software, the same team that brought you the excellent 1999 Half-Life expansion, Opposing Force, in which you play as one of the commandos sent in to ventilate Gordon Freeman, the protagonist of the original game.
It's worth noting that Half-Life's publisher, Sierra Studios, has used the Half-Life brand name on many other products, including refurbished 'Platinum' and 'Game of the Year' editions of the original game, and has also helped promote (and sell) fan-made Half-Life mods, including last year's Gunman Chronicles and the extremely popular multiplayer mod Counter-Strike.
Half-Life: Blue Shift is the second official retail expansion to Valve Software's groundbreaking 1998 first-person shooter.