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.
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Pro Very flexible
Org-mode is characterized by a flexible and versatile system with adaptability to different workflows, making it comparable as an Evernote alternative. It is at once simple and complex., which helps it to compete as an Evernote alternative.
Pro Great sync support
Notes and to-dos can be synced with Trello, Toodledo, Simplenote, Orgmobile, or with tools like Git on Github, Dropbox, Google Drive, and Bitbucket, while WebDAV (with iOS and Android) can also be exported to PDF, mind map, LaTeX, HTML, Docbook, or txt.
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 Built-in agenda
Org-mode has some built-in agenda functionality. You can schedule tasks and assign various degrees of importance to each of them. Org-mode agenda can also be synced with Google Calendar.
Pro Versioning can be tracked and synced using Git or other VCS
Org files are plain text, and lend themselves well to version control. Emacs also has good integrations for various VCS.
Pro Out-of-the-box Latex support
Org-mode has out-of-the-box support for Latex: it can immediately parse equations (or other Latex markup) and can even compile notes to PDF or HTML.
Con Difficult to learn
Org-mode has a difficult learning curve since you have to learn all the keybindings and commands. It's especially difficult if you are not used to Emacs.