Neither of them are free.
I got the second one on a sale for only $5.00, but the first one's sadly not around the same price -- the second has a few campaigns that I've heard carry over from the first, so I'd just as well get that one -- it's a really nice game when you're not lagging
Torrents.....yeah, they're pretty much useless for all Valve games -- the multiplayer is always the best part, so torrenting things like TF2 and L4D are useless unless you want stupid AI partners.
If you don't know where to download steam, check it out on Google and make a account, and download the program. A simple search for 'Left 4 Dead 2' in the program should give you a copy, and if you can buy it, everyone'd be glad to play with you. :3
But, yeah, it's honestly my favorite game on Steam currently. Zombie hunting and goofing off with your friends is a good formula and it hasn't died yet....eagerly awaiting Left 4 Dead 3 if they get around to it...~