Org-mode is for keeping notes, maintaining to-do lists, planning projects, and authoring documents with a fast and effective plain-text system.
SpecsUpdate
Question Ranking
Pros
Pro Files are usable anywhere at anytime
Users are not tied to one service provider, program, platform, or database engine.
Pro Ultimate flexibility
This app's flexibility is based on its minimalist approach, giving the user near-infinite freedom.
Pro Agenda views
Pro There are a lot of extensions, for exporting to html, bootstrap, js-reveal and much more.
Pro Absolutely free
Emacs with Org-mode is free as in beer and free as in speech – that is, it costs nothing and it’s totally open source.
Pro Incredibly extensible
There are many plug-ins for Org-mode, including Org-habits and Org-notify. If Org-mode lacks some piece of functionality, it is very easy to add it.
Pro Supports plaintext spreadsheets
Pro Efficient features for deadline organization
Pro Quickly add rich text
Pro Excellent unofficial Android app (orgzly)
Cons
Con Android app isn't very good
There are several user-created apps for Android, but none seem to offer the same level of functionality as other to-do apps.
Con By default, a hard-to-read display
The default way of writing an outline or checklist creates a very messy wall of text that's difficult to read with no vertical spacing. You can manually add vertical spacing, but the Org operations don't preserve it. There are pretty-display modes, but you need to remember how to enable them, etc. etc.
Con Unintuitive user interface
The key combinations are unintuitive and difficult to remember. This is probably because there are a lot of hidden "modes" depending on where the cursor is. Actions aren't paired with reversing actions like in other todo apps. For example, hitting shift-tab does NOT reverse the effect of hitting tab.