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Edit: many people are noting that the comments and reviews are out of date.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.