"I'm VERY surprised," says Minecraft forum member Xannot, "that nobody has recreated Half-Life in Minecraft yet." So he decided that he'd do it himself. Here's the result, sadly lacking the tram ride introduction, but running up until your first encounter with those bizarre alien dogs.
You can download the map here and you'll need one of the two latest snapshots, either 12w40b or 12w41b. Don't worry about the blocks around the villagers, ahem, the scientists, as those are only there to stop them wandering off. Yes, I suppose they do ruin it a bit, but in time an update should render them invisible.
Look familiar? The map's fairly large. If you need help getting around, be sure to use the signage.
Ah, it's all coming back now.