for the love of god get team fortress 2!! its free on steam and AMAZING! over 70 maps and just great fun
another wicked free game on steam is Hawken,,,its a fps mech combat game and its AMAZING fun and one of the best free games on steam,,i don't really like mech type games but love this
dirt rally is GREAT and a really fun challenge,,,not free but worth the cash imo,,you can get it cheaper off a key site
battlefield 4 is wicked as well,,,loved it on ps4 and love it more on pc,,again not free and can be bought cheeper from a key site,,its on origin not steam
oh nearly forgot,,arma 3 is also AMAZEBALLS!! I had a "cough captain jack sparrow cough" version lol

but I'm defo going to buy it,,,its prob the BEST sim fps out there!