Re-enter the Half Life Universe and revisit City 17 in the time before Freeman, in this fan made single-player mod. The Resistance is threatened by a decisive Combine assault, and as a wanted Resistance member you will need to fight to escape from the Combine's clutch. At the same time, a mysterious force is working towards ends unknown...
This is quite possibly the best single-player HL2 mod I've played - especially the first 10 minutes or so. As others have mentioned, there aren't really any new design ideas that you haven't already seen in the Half-Life series, but the quality of the maps is right up there with what Valve has produced. It would be nice to have a more exciting battle for the finale though, as the current ending seems a bit anticlimactic.