Definitely the best and biggest website if you want to win steam games for free. The Steam account you use for registering must have $100.00 worth of games to prevent alternate accounts. Read the FAQ carefully if you'd like to join this site! The system that chooses a winner is completely random, but if you'd like to increase your chance in winning and you intend to give away games yourself at a regular basis, look out for private steam groups that require giveaways each month from all members (the less members the higher the winning chance).
This one is free as well, but for better winning chances you can either give away games yourself or spend money. The Steam account you use for registering must be valued at a specific amount to prevent alternate accounts. Winning on this site is easier if you are good in minigames like Bejeweled and Bubble Shooter, since you have to play them when you enter some of the giveaways. All games given away there are Steam gifts, you can't win or give away keys there, but you can even enter for games you already own! As always, read the FAQ if you got more questions.
Play browser games for free and get achievements and coins, until you get enough coins to buy steam, origin and desura games with them, or alternatively complete surveys and offers for a lot of coins! You can enter giveaways with them as well! This is one of the best ways to not only get games but also Bitcoins, Amazon coupons, Steam Trading cards and much more for free very fast!
News: They host new giveaways every 4 hours now!
A russian site (that has been fully translated into english now) where you can win steam games for free, you can even enter for games you already own! You can buy better chances of winning for money, as well as on Playblink (or alternatively give away games to be able to enter specific giveaways).
You don't even need an invite code to join the site anymore!
As always, read the FAQ to know how the site works.
A french website that is still in BETA (they are working on translations etc), it's similar to steamgifts. You get points when other users create giveaways for steam games and can spend those points to enter giveaways, it's completely free. There aren't that many giveaways yet because it's a new website.
Among various other features SteamCompanion offers a game giveaway system very similar to steamgifts. You get points that you can use to enter giveaways for games, created by other users, completely for free. For a specific explanation on how to get points look here, for everything else read the FAQ.
One of the various things this forum offers is a giveaway section for game keys and gifts. You can register for free, most of the time you have to reply in a specific thread to enter a giveaway for a game. Note that these forums also contain NFSW stuff so browse it on your own risk.
In the first thread all the games that are available for free on Steam are listed.
In the second one ongoing giveaways, promotions, Betas and contests you can find around the web are listed. It's a good source for these kind of news.
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