There was a question from Dave Froslie on what we use to store our Product Backlog. We use an internal wiki page. Wiki’s are nice because they’re easy to view, easy to edit (not only for the Product Owner but also for people with suggestions who can add them in a suggestion section at the bottom for the Product Owner to prioritize later), and you can easily see different versions. You can also track modifications and hunt down people who mess with prioritizations.
For our Sprint Backlog we use an (ever more elaborate) Excel spreadsheet.