Ranked in these QuestionsQuestion Ranking
Pro Cross platform
Emacs runs on all platforms, Windows, Linux, OSX, even Android and IOS
Pro Libre/open source
Pro Exports data to PDF, LaTeX, HMTL and other
You can even build presentation or blog from your data
Pro Can be as simple or as complex as you want
Org mode started out as a simple outlining/note taking app. Then each outline heading can optionally be a TODO list item. And if you desire, you can add SCHEDULE and DEADLINE dates. (Schedule being the date you'd like to start the item.) You can customize the TODO states to add things like DELEGATED, WAITING, BLOCKED, etc.
Pro Supports tables with formules (quite powerful!)
Pro Inline image displaying
Pro Plain text data storage
Pro Uses Emacs
Pro Free but copyrighted
Con Obtuse documentation
The documentation assumes a level of understanding of basic Emacs usage that not all users may have.