efaq: Packages that do not come with Emacs
8.3 Where can I get Emacs Lisp packages that don’t come with Emacs?
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The easiest way to add more features to your Emacs is to use the command
‘M-x list-packages’. This contacts the GNU ELPA (http:///elpa.gnu.org)
(“Emacs Lisp Package Archive”) server and fetches the list of additional
packages that it offers. These are GNU packages that are available for
use with Emacs, but are distributed separately from Emacs itself, for
reasons of space, etc. You can browse the resulting ‘*Packages*’ buffer
to see what is available, and then Emacs can automatically download and
install the packages that you select. 
(emacs)Packages.
There are other, non-GNU, Emacs Lisp package servers, including:
MELPA (http://melpa.org/); and Marmalade (https://marmalade-repo.org/).
To use additional package servers, customize the ‘package-archives’
variable. Be aware that installing a package can run arbitrary code, so
only add sources that you trust.
The GNU Emacs sources mailing list
(https://lists.gnu.org/mailman/listinfo/gnu-emacs-sources), which is
gatewayed to the Emacs sources newsgroup (news:gnu.emacs.sources)
(although the connection between the two can be unreliable) is an
official place where people can post or announce their extensions to
Emacs.
The Emacs Wiki (http://emacswiki.org) contains pointers to some
additional extensions. WikEmacs (http://wikemacs.org) is an alternative
wiki for Emacs.
The Emacs Lisp List (ELL)
(http://www.damtp.cam.ac.uk/user/sje30/emacs/ell.html), has pointers to
many Emacs Lisp files, but at time of writing it is no longer being
updated.
It is impossible for us to list here all the sites that offer Emacs
Lisp packages. If you are interested in a specific feature, then after
checking Emacs itself and GNU ELPA, a web search is often the best way
to find results.
Info Catalog
efaq: Finding a package with particular functionality
efaq: Finding Emacs and related packages
efaq: Spell-checkers