If you mean, where can you get free keys to games on Steam, unless you get them gifted to you (from a friend or from a developer in exchange for a review) or as part of a bundle, I strongly discourage you from investigating. These “free key” sites are scams, looking for either credit card numbers or personal information that they can use to phish or socially engineer you into revealing more, then engage in identity theft, or install malware for who-knows-what purpose.
If you’re just looking to freeload off of someone else’s hard work, I ask you to reconsider. Game development isn’t cheap, and for smaller shops it requires a great deal of personal sacrifice in the pursuit of creative expression and entrepreneurship. Please consider supporting them.