A customer of mine had the need to get an overview of all the checked out documents for a given Site Collection, and to check them in afterwards prior to a content migration from SharePoint 2013 to SharePoint Online.
Since no out of the box features exist to accomplish this goal, I had to write a Powershell-script to find all the checked out documents in a Site Collection.
You can find the script here. It works for SharePoint Online as for SharePoint 2013. I did not test it with SharePoint 2010.