You could look through your package manager's dependency graph, to find those projects which are more "core" (i.e. those which are depended-on by the most things you use).

When Ubuntu added paid-for software to their package UI, there were discussions about making it easier to pay FOSS developers too, but I don't think it got anywhere:

https://wiki.ubuntu.com/DonationSystemSpecification

https://wiki.ubuntu.com/SoftwareDonations