Did you know that you can play a single player game of NS? I was reminded of this 'bug' in the HL engine by Flayra's post of the oldest NS screenshots which showed marines in the Black Mesa map.
So, how can you play NS as a single player game? Simple. Activate the custom game NS. And either start your own lan game or just open the console. In the console use the command 'changelevel' to change the map to one of the single player maps. It should now start you in one of the regular HL maps and if you are lucky all of the scripted sequences in the maps should work.
I haven't tried this trick in a while so the procedure might be a bit off. As soon as I reinstall NS (new 120gig hd ) I will test out the exact procedure you need to follow to accomplish this task.
You will also need to know the cheat commands because HL is easily confused by a multiplayer game and you could end up spawned in a wall or other unlucky place.
The following mods are also fun to try out this trick: DoD and RocketCrowbar. DoD is especially odd when watching Gordon Freeman running around with an M1 Garand
Try it out and let me know if you get it to work with NS.