Link list

Homepage
Webseite (deutsch / German)
musescore.com community site (allows score sharing, group discussion, etc.), a freemium add-on service
Downloads (MuseScore itself is Free Software under the GNU GPL)
Official PPA for stable MuseScore releases for Ubuntu and derivatives (Linux Mint, KXStudio, …) users
MuseScore in Debian (with newer versions in official backports)
MuseScore Apps for mobile, freemium add-ons allowing playback, with (limited) options for per-track volume control, metronome, transposing, sizing, rendering, for iPhone and Android/Kindle (Play Store version also works with MicroG)
FAQ
Plugins (documentation)
OpenScore — Liberating Sheet Music, project to publish digital editions of Public Domain classics under CC0 (mostly using IMSLP as source), in MuseScore format, but also exported as MusicXML, Braille musical notation, …
The OpenScore collection of liberated music, on the musescore.com community site

MuseScore online manual (English)
MuseScore online manual (German) / Musescore Online-Benutzerhandbuch (deutsch)
Mastering MuseScore is an actual, published (paper and Kindle) book covering the material from the official handbook and some day-to-day help, written by Marc Sabatella, MuseScore contributor and educator

English MuseScore Forums, official discussion and support on the main platform
deutsches MuseScore-Forum, offizieller Ort für Diskussionen und Hilfegesuche (German)
MuseScore-Unterforum im Notensatz-Forum (deutsch / German)
Notations-Unterforum im Musiker-Board (deutsch / German)

MuseScore tutorials (mostly videos) on the official site
MuseScore HOWTO (text with pictures) on the official site
MuseScore-„Wie wird’s gemacht?“-Anleitungen (bebilderter Text) auf der offiziellen Webseite
MuseScore HOWTO video tutorials on YouTube, by the MuseScore team
MuseScore Café, weekly live interactive chat session, by Marc Sabatella, on YouTube
Quick step-by-step answers to common questions, with videos, also by Marc Sabatella, on his Mastering MuseScore Online site (which also offers paid-for and free courses, which, in addition to the Mastering MuseScore book, are explained here further)
MuseScore Educational Partnership: Making MuseScore an integral part of music education; partnership for both educators and students
Resources for music notation/engraving standards and conventions in the official Forum
IMSLP Petrucci Music Library — large collection of Public Domain music
free-scores.com French/English score sharing website (both libre and licenced)
more links to Free Sheet Music

Send scores to YouTube from the musescore.com community site, or even synchronise a score video with a real performance audio/video
How to use MuseScore with Hauptwerk and Grand Orgue Virtual Pipe Organs details MIDI and score-level changes to properly support organ registration as well as setting up those applications to work directly together
The Hauptwerk or GrandOrgue users group on the musescore.com community site contains tons more hints in discussions and training scores for proper organ typesetting (the “Pipe/Church Organ” instrument isn’t suitable, it has a fixed Mixtur registration — “Reed Organ” is supposedly better poison for using unregistrated sounds)
Arduino + LEDs + MIDI keyboard + MuseScore = Piano Tutor hacking project (announcing tweet, demo video, hack video, tutor video, Instructables, MuseScore software patch with Arduino code in miditools/PianoTutor/)
In the MuseScore Developers’ Handbook, the section documenting the instruments.xml format contains some information on setting one or more MidiAction elements that can be triggered from a score to send arbitrary MIDI CCs (here is an example)
My slides from this workshop