Here we'll keep a list of resources specific to Mixed Method systems as they look different(ex. BuJo in a Happy Planner isn't the same as a BuJo so to speak). I'll update the main list with any recommendations posted in response to this thread.
Evernote [multiple workbooks with notes; free version limits number of devices, paid version add video clips, use on more devices, etc.]
Basecamp [handy program, good for scheduling tasks/to-dos, receiving notifications; used to organize/set-up RP group along with GDocs]
Google Drive [Between the ability to share docs/files, create folders, etc. it's quite useful for organization]