http://www.dayofdefeatmod.com/
Day of Defeat is a very fun WWII Source mod. I've tried many source mods that I've come to hate, this one is definitely a good one
not to hate. The gameplay is rather hard, when you shoot your gun the recoil is as if a 7 year old is trying to fire a Assault rifle, which is pretty gay but easy to adapt to. The sniper is almost aim in the general direction of someone while unscope and shoot which is also another perk that kind of brings the game down. Machine Gunner class is impossible to play unless you have it mounted our are crawling on the ground. Those are really the only 3 bad things about it. It's very fun once you get used to it. I was doing pretty well the first time I started playing after adapting to the burst shot fire. The game is very fun and very tactical, meaning, you won't just rush out in the open and get a quadruple kill. The ragdolls make it also really laughable. What's funnier than throwing a grenade over a wall then seeing a nazi's body flip everywhere while sailing through the air over your head?
The game is $10 through steam and requires that you have a source game (Preferably HL2). I recommend getting the Orange Box http://store.steampowered.com/sub/469/ It's $30 so if you don't want to spend so much money you can just buy HL2 for $20 (Would just be better to buy the Orange Box in whole because it would be a lot cheaper that way, plus TF2 is pretty fun). It will come out to be at $40 or $30 depending on which you choose, I was skeptical about buying DoD: Source but was glad that I bought it because it's very fun. Put it into consideration.
https://www.youtube.com/watch?v=8BRYhg7MKec
Some gameplay action, didn't know that Valve made it, explains why it's so good.