Lost Coast is free, just browse the games on Steam on you'll find it listed and free for you to download (if you own HL2).
It's interesting. Picture the director's commentary of a movie and that's what LC is, except it's HL2.
Basically, they created a "level" that you do play, but you also can stop and listen to the developers discuss game design and describe exactly what is going on with the game engine during various effects and such.
"Happy slaves are the worst enemies of freedom." - Marie Von Ebner
"It was always the women, and above all the young ones, who were the most bigoted adherents of the Party, the swallowers of slogans, the amateur spies..." - Orwell
The Riddick game's "Developer's Cut" had the same function, but it was a little hard to listen to commentary when you still had to fight your way at the same time through the game.
"Be bold, and mighty forces will come to your aid."
Lost Coast has markers on the map that you can stop at to activate specific audio clips. This is designed so you're in front of whatever they're talking about. I'm pretty sure it's possible to avoid any action while listening to these clips.
"Happy slaves are the worst enemies of freedom." - Marie Von Ebner
"It was always the women, and above all the young ones, who were the most bigoted adherents of the Party, the swallowers of slogans, the amateur spies..." - Orwell