This is a collective list of large trusted websites that offer great deals on Steam games. If you're looking for a great deal on Steam games, take a look at what these sites offer. These websites are 100% trustworthy and verified to work.
List of Trusted Stores:
Amazon.com: Amazon always has great deals on Steam games and lets you sort deals by the percent off.
Steam Store: Self explanatory, but the Steam Store is the best place to find steam sales. Duh.
Humble Bundle: Website that sells both Bundles and individual games. Don't forget to check the Humble Store as well!
GOG: Owned by CDProjektRed (The people that made the Witcher), you can find great deals on this site on DRM-Free games.
GreenManGaming: High quality key reseller that always has popular steam games on sale.
Newegg: Newegg.com is an online electronics/computer part store that also happens to offer Steam Games for sale
Origin: Not exactly Steam, but Origin offers great game deals sometimes, especially on Battlefield games.