If anyone knows good sites to get games cheaper than steam prices, I'm not very sure which one's are trustable. Reason for asking, look at CoD games on steam. (Not the best series of course.) But Black Ops 1 is still 40 bucks, and black ops 2 and 3 are both 60. Not that anyone in their right mind would get Black Ops for 40 bucks, those prices are ridiculous. I already have a library I can live with for quite a long time, so there's no urgency, but as a future reference, it would be helpful.
I'd recommend allkeyshop.com. For each game it gives you a list of websites with their prices. In terms of specific reliable websites, you can never go wrong with Kinguin, cdkeys.com and G2A, but they're not always the cheapest.
I know friends who used Kinguin, and it may not be much cheaper unless it's on sale, but Humble Bundle is nice too. Buy a game, give a portion to charity. Seems like a win win for me. Save money and help others.
Like other already said, I really recommend Humble Bundle. They have different games on sale almost every week. And when you're looking for a specific game, I recommend checking on isthereanydeal. And I really do not recommend those "key-resellers". While most of the time you get your key and it's working, it's still very unsafe, since your key can always be revoked by Steam when they find out through which means you bought it. Most of the time, those keys were bought with stolen credit cards and that's why they're resold for such a low price. I'd always stay clear from those shady key-resellers like Kinguin, G2A, and whatnot.