We use Scrum with some constraints. Each subteam is in charge of a specific "product" and manages its specific product backlog. Product backlog items are usually user stories described using the popular "As an <actor> I want to <action> to achieve my <goal>" form.

Each subteam is made of 5-9 contributors and is working on weekly sprints. Given the constraints of volunteering and partial dedication of team members, Scrummasters rotate and we use conference calls for combined planning and review meetings. There are no daily scrums, but almost daily emails exchanges.