Here are some that I have found useful:

Game tracking:

• HowLongToBeat - Track your backlog, see how long a game might take to beat
• Retro Achievements - Achievements for retro games

Game art:

• SteamGridDB - Game art banners etc. to personalise your Steam library
• Steam Game Covers - Box Art for Steam games

Compatability

• Does It Play? - Lets you know if a game plays out of box or requires extra downloads or codes
• Proton DB - Compatibility ratings and advice for gaming on Linux

Game Deals:

• Fanatical - Can often find games cheaper here
• Humble Bundle - Does game bundles with proceeds going to charity, often has good games in the bundles.
• CeX - Can pick up second hand physical games for cheap here
• GOG - DRM free games