Actually the MultiPlayer aspect is "FREE".
It's called F.E.A.R. Combat and is one of the most customizable i've seen in years. Almost as customizable as Q2. There is a massive amount of MODS and custome crap for this game. Plus as an added feture it will really get down with some great smack talk to, which brings me to the "Duke Nukum mod" Hale to the King Baby !!!
You can download it here.
http://www.joinfear.com/mainNow if you want the single player aspect aswell then yes, you do have to buy it. And YES IIIIT IIIIS WOOOOORTH IT !!!

One of the best FPSers EEEEEEVER. It is the ONLY FPS game that has ever ever been able to pull me away from my beloved Quake 2.

The AI is brilliant aswell. They can see your flashlight, hear you make noise, see you in the dark.
Is that an empty 5Gal paint bucket, carefull, don't knock it over, they'll be on to you.

It is also very creepy, that little ghost girl creeps you out bigtime..
But, on the other hand if I just want to relax it's a nice to just go around and bust out windows, reck computers, snipe the rats, etc . . .