Recs.
Updated
Org-mode is a note taking tool to be used with Emacs. It is geared towards authoring documents, taking note, keeping todo lists and planning projects, all in a simple text format.
An online editor that has some Org-mode support is markup.rocks.
Specs
Pros
Pro Easy to use as its plain text
It is plain text which visible in any text editor. Out of the box support is best in Emacs as of now.
Pro Basic spreadsheet functionality
Org-mode has some basic spreadsheet functionality. Other than auto-formatting ASCII tables (a notoriously annoying problem), it also has support for LISP-like syntax to define equations or any other functionality that can be achieved through spreadsheets.
Pro Regular syntax for exporter-specific adaptions
To polish output for a specific exporter — i.e. LaTeX or HTML — you can define content that only that exporter sees. This enables the small tweaks that make the difference between just a conversion and getting a beautiful document in different formats, using the strengths of each export format instead of limiting to the smallest common denominator.
Pro Expands beyond other markup tool functionality and can work with mobile applications
Pro Embedded metadata
Orgmode uses a number of metadata values which can be defined inside a document such as author, date, document title etc. which can be but do not have to be used by different exporters. These can be especially useful when searching through a large number of files/documents or documenting the evolution of one as part of a larger codebase.
Cons
Con Limited support outside of Emacs
If you want to use a different text editor, you miss out on advanced features.