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Or just C-h R emacs/elisp/eintr/... RET in your Emacs.
More than a mere extension language.
Themes built-in or from (M)ELPA.
M-x shortdoc cheat sheet.
A Git Porcelain inside Emacs.
Organize your life in plain text.
Keep track of your notes.
The Emacs Calculator.
Remote file editing.
The Emacs Shell.
An extensible vi layer.
In case of a non-libre OS.
All the *info* manuals bundled with Emacs.
