[stephenr]

Specific sponsorship ie a company sponsoring a project and having mention/logo/link on a readme/sponsors page etc I think is quite acceptable.

A random Google ad (particularly with the privacy invasive tracking that goes with it) on the page is going to give an altogether different appearance, particularly if the project is using free resources such as GitHub/read the docs/etc.

[Gammarays]

What if Google wanted to support an Open Source project. Would it still be appropriate if the project mentioned Google as a sponsor, then showed a banner ad promoting the new Google Phone (no tracking/banner does not change)?

[stephenr]

Saying its supported by Google and giving a link to google.com would be fine. Showing an actual advert I think is not fine.