Roll20 Con is on towards the end of this month, and they're giving away free modules.
So far Alice is Missing and Apocalypse Keys have been announced and are available.
They still show in the marketplace with their price tag but have a "Claim your free module!" button when you go to their store page.
Direct links:
Alice is Missing: https://marketplace.roll20.net/browse/module/6051/alice-is-m…
Apocalypse Keys: https://marketplace.roll20.net/browse/module/18274/apocalyps…