Plone is free to use, but built, maintained and improved by someone in their voluntary time.
Rockstar Plonista @ericof had this great idea 8 years ago - a birthday fund raiser.
Come on, contribute!
Sponsor the upcoming Plone Conference 
Sponsorship
Sponsor the Plone Foundation Sponsor Plone
Donate via GitHub Sponsor @plone on GitHub Sponsors · GitHub
