You can keep notebooks stored in plain-text and have them synchronized with a directory on your mobile device, SD card, Dropbox, or over WebDAV.
Create, edit and delete notes and tasks - Collapse and expand sub-notes - Schedule tasks and set their deadlines - Set priorities and tags for notes - Fling notes left or right to open quick-menus - Search notes by various properties - Create and save searches for quick access - Synchronize notebooks with one or more directories
Notebooks are saved in Org mode's file format. "Org mode is for keeping notes, maintaining TODO lists, planning projects, and authoring documents with a fast and effective plain-text system." See http://orgmode.org for more information.
Simple but capable. You can sync local repository with Dropbox as a backup.
very easy to use for org-mode on android to open and edit documents, tasks, todos in org markup language.