You may renember that Bukkit was sold to Curse and later bought by Mojang.
There were Curse logos on all Bukkit websites, they managed the forums, displayed advertisements, etc.
Now it turns out that the Sponge project is also “sponsored” by two companies. From spongepowered.org:
I want to know:
Who exactly is the owner of this websites?
Who can access the database tables of this forum?
Who is the owner of the sponge code?
What does sponsorship mean? Is there a contract, can we leave this sponsorship?
I would say whoever the systems admins are, whomever owns the box it is hosted on (technically), and whoever the web designers are (See 1)
The Sponge Code is licensed under MIT. In so much as there are owners, it would be the public.
a - Sponsorship generally means that in exchange for hosting, resources, or something similar (in this case I am assuming web hosting & bandwidth) you generally agree to display the logos/links of the sponsors in prominent places.
b - Almost all sponsorship involves a contract of some sort. What the details of that contract are, only the signing parties know and they aren’t required to tell us. I’m sure there are clauses in it for dissolution of the partnership by either party though. As a user you can leave it by not using the site.