Live instances

Public

These are public sites running GNU MediaGoblin:

http://roaming-initiative.com/mediagoblin/ running on http://hyperboria.net/ apparently! reddit thread
http://mediagoblin.com -- Currently registration and video are disabled; you have to ping Chris Webber for an invite.
https://efjz.in -- "EricFontaineJazZmediagoblIN" is for me to archive recordings of music performances of me and my fellow musicians.
https://goblinrefuge.com -- Registration open. Images and video enabled.
https://mediagoblin.pixelminers.net/ -- Fast and secure (HTTPS enforced) Mediagoblin server. Open, free for all.

https://gobblin.se (previously mg.wandborg.se) -- Registration open, but uploads are currently broken (help fixing is welcome!); all media types enabled (except STL/3D, at the moment) -- details; Joar/mg.wandborg.se.
Your browser may not trust the certificate authority: http://wiki.cacert.org/InclusionStatus.
http://barn.kerosenecow.net/ -- Registration disabled. This is mainly a showcase for development of the MG-GIMP project (a Mediagoblin plug-in that permits hosting of GIMP images and resources).

http://isengaara.de (not running as of 13 Oct 2014) -- IsengaaraP's mediahub for sharing eCantorix songs and コスプレ photos.

These are instances run by organizations, usually having registration limited to organization members:

http://medii.ceata.org -- Fundația Ceata, Romanian Free Software and Free Culture activist organization. Registration is currently disabled.

http://media.libreplanet.org -- Videos of the LibrePlanet 2013 conference (Free Software Foundation). Registration is currently disabled.

http://media.openvideo.pro -- Open Video Productions, L.L.C. Registration is currently disabled.

http://media.chicagolug.org -- Chicago GNU/Linux User Group. Registration is currently disabled.

Also, these sites are running MediaGoblin, but are for more limited audiences (family/friends of person who's running it, etc):

haise family photos (currently with a baby focus)
jpope's instance -- Registration currently disabled.
thelovebug's instance (registration closed)
photos.katsarov.org Photos by Anton Katsarov (registration closed)
media.micha.elmueller.net Photos by Michael Müller, all licensed under CC-BY (registration closed)
mediagoblin.aurka.com janssen’s personal mediagoblin-instance with concerts, demonstrations and other stuff (registration closed)
media.deadsuperhero.com Artwork and photos from Sean Tilley, who writes MediaGoblin themes for fun.
media.chrishancockart.com Artwork from Chris Hancock. (registration closed)
photos.technicallyandrew.com Photos by Andrew Browning (registration closed)

maiki’s baby photo site (can't connect on 16 Oct 2014)
spaetz' private pics (under construction on 16 Oct 2014) Sebastian Spaeth's pictures, family and stuff (registration closed)
gallery.Dick-Thomas.co.uk (can't connect on 16 Oct 2014) Dick Thomas' pictures, family and stuff (registration closed)
media.wigunnara.com (can't connect on 16 Oct 2014) Giri Alam's pictures, family and stuff (registration closed)
noreal.info/galeria (does not open on 16 Oct 2014) Run in a Raspberry-pi by Arder Vegan (registration closed)
ardervegan.info/media (error 404 on 16 Oct 2014) Run in a Raspberry-pi by Arder Vegan (registration closed)
[1] (can't connect on 16 Oct 2014) Pictures and music from Charles Pehlivanian, hosted on a Raspberry Pi.
[2] Laura Arjona's pictures and videos, family, and libre software stuff (registration closed)

Indyshop:Fashion Blog & Independent Shop Showcase - Registration by email request for independent brick and mortar shops only.

You can add your instance here, too!