As I like to source my own ROMs, those 4 tend to be my primary places to look:

• https://www.gog.com/forum/general/roms_obtainable_on_gog_compendium/page1

• https://itch.io/games/ (can filter by platform-specific tags for the homebrew ROMs)

• https://retropie.org.uk/forum/topic/10918/where-to-legally-acquire-content-to-play-on-retropie/1?lang=en-US

• https://github.com/farmerbb/RED-Project/wiki/

Also other places, but most way smaller and mainly found through the 4 above.

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

time extension!

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

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

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.)

dekudeals, howlongtobeat, xbdeals.net, and

https://retroachievements.org/games

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

Vimms lair, of course.

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

Gaming on Linux - for news

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

http://www.hardcoregaming101.net/

https://racketboy.com/

Edit:

Great question @B0NK3RS@lemmy.world , great thread, thanks