Is there an app where to mark games that I want to play and have played? Like goodreads but for games?

I use https://www.criticker.com/ for movies because it has a really nice recommendation algorithm based on your personal scores. They also have a section for rating games but I haven't tried that part

https://gamefaqs.gamespot.com/boards/189706-nintendo-switch/79890338

I used Backlogery, but Backloggd seems to be more popular nowadays. That forum discussion mentions a few others, too.

If you play retro games, then you could also look into RetroAchievements, which will track more than when you simply beat a game. Unfortunately, it doesn't work for all games though.

For more modern games, you could also use Achievements to track what you beat.

As for keeping track of what games you WANT to play... it's called buying them! Don't buy games you don't want to play, haha

Seriously though, you could go through your Steam library and add games you want to play to a playlist, or sell physical games you wanted to support but don't want to play.

But for multi-platform stuff, check out the linked forum post

Commenting so I can come back to this later with the site, I can't recall the name at the moment

Alrighty, it looks like the list has grown and I can't remember what site I had used previously, so here are a couple options. It looks like they all roughly have the same format of: create account, fill out games from database, possibly account and app linking options.

In no particular order:

How long to beat: create an account, has a games library for your profile

Keep track of my games: create an account, "pay what you want"-ware (free), can import gaming accounts (Steam PSN etc) to fill out list.

Backloggd: Create an account, can fill out games to your library and has space for reviews and other user profiles

Grouvee: Create an accout - homepage is pretty minimal

Gametracker: Seems more "game team" oriented but it has a spot for filling out a games library

GameTrack: Has an IOS app as well, can link gaming accounts for achievements, can make lists to sort games

Playtracker: Create an account, looks like there is a software download for the computer

Stash: Has both Android and IOS apps,

Of all of these, the feature sets look basically the same, the main differences seem to be UI layouts and more niche options of sorting/filling out. All of them look to need an account (expected). Since I can't recall which, if any of these, I had used in the past I will just say that the websites for Playtracker, Backloggd, and How Long To Beat looked the "best".

Hopefully this helped and didn't just give you more choice anxiety, lol.

There are quite a few, as listed in another comment here. I've been using backloggd for years now and love using it!

Shame there isn't an ActivityPub/Fediverse option...