The Orange Box is a masterpiece. It costs 14 euros on Steam right now, and that's without being on sale. That's MENTAL value for your money. Everyone who doesn't have it already should pick it up.
You ever played the original Half-Life? If not, it's not a bad idea to play that one first, to really get to know the storyline. It's not all that important, but something I would recommend.
One of the best gaming deals ever. Even the first Portal is an awesome game. I haven't played the original either. Will definitely look into that as well.