AlternativeTo is a site I use quite a bit. Personally I use it when I get fed up with an Android app having too many ads / creepy network behavior or want to find a self-hostable version of a freemium service.
It has filters for free, open source, platform type, etc. From my understanding it's all crowd sourced, so if you disagree with a rating put in a vote! Sharing this in hopes that others find it as useful as I do.
If you know of similar or better resources I would love to hear about them.
Edit: many people are noting that the comments and reviews are out of date. I agree! Despite that I still find it to he useful. It would be great if this little bit of visibility gets more folks engaged over there to improve it.
I always thought this and all the other sites like it were just SEO bait. The quality of the alternatives is no better than random, though I guess you can discover software you didn't know existed. One example: I'm always looking for an alternative to Roon. Like Plex for music, plus you can add in tidal or qobuz, so you can essentially listen to anything on your own library, or the streaming services, from anywhere. It also has an incredible metadata system, letting you view for example, all the players on a jazz album, view bios of every one, and easily start to check out some new music that way. It's unbelievably flawed and poorly run though.
What is alternativeto.net's suggestion as a replacement? Fucking VLC Media player. The only real contender is PlexAmp, or if you want something similar but only for classical music (and without self hosting your own library), there's idagio. results like that make me think it's totally useless
This is also my experience. The results are usually very vaguely related with what I search. I had better luck with Wikipedia comparison tables, but often they are incomplete.