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

yucata.de

Boardgames online, excellent!

Asynchronous move timing, you can take up to 29 days to play a move.

Community built, DIY ethos, free to join, donation supported.

Gaming on Linux - for news

Hardcore Gaming 101 - when I want to go down a rabbit hole of game series history and trivia

dekudeals, howlongtobeat, xbdeals.net, and

https://retroachievements.org/games

I mostly use nintendolife. That's a Nkntendo only website. Back in the day I enjoyed Destructoid but that became utter trash.

https://www.steamgriddb.com/grids
For all of your artistic game cover needs.

Vimms lair, of course.

And
https://metroidconstruction.com/
For Metroid hacks.

The ROMsMegathread to add to the other recommendations. Also I do have to mention GoG again for DRM free games.

I'm a big fan of https://shmuplations.com/

They translate or host a bunch of first-hand interviews with developers of classic Japanese games. (Not restricted to shmups, despite the name.)

Another extremely useful one to me is https://www.doesitplay.org/

I'm not OK with games requiring a download. And developers don't always do a good job of making it clear when they do. Or a handful of games might be nearly unplayable in the version that ships on disc/cart. This website makes a point of testing games unpatched and offline.

It's not a big deal for everyone, but the site is a crucial resource if you care.

libregamewiki.org

Open licenced games and media.

I've played through most of the ones older than 7 years old.

Here are some solid start points:

Neverball, Kobo Deluxe, Project: Starfighter, Frozen Bubble, Hex-a-hop, SGT Puzzles, Numpty Physics, Fish Fillets NG, Enigma, Passage, Shattered Pixel Dungeon, X-Moto, Blobby Volley 2, Stormbaan Coureur, GCompris

https://maxutmost.com/

time extension!

All I need are Steam, GOG, Isthereanydeal, Howlongtobeat and SteamDB. There are some YouTube channels I follow, but as they mostly talk about consoles nowadays, I don't really rely on them for the final choice.