If you are looking for software, a great site to start is at Alternative To. There are a few ways to go about searching on the site. One way, if you already know of a similar product is to search for it, then look at the alternative matches (which you can then filter by license and platform).
The other option if you don’t have anything in mind is to simply browse by platform and license across all software. You can use the most likes/views to see what’s being used.
A very handy site and a comprehensive database.