Keyboard shortcuts can be customized via Edit → Preferences... → Shortcuts tab (Mac: MuseScore → Preferences... → Shortcuts tab). Below is a list of of the initial shortcut settings.

Navigation

Beginning of score: Home
Last page of score: End

Next page: Pg Dn
Previous page: Pg Up

Next measure: Ctrl+Right (Mac: ⌘+Right)
Previous measure: Ctrl+Left (Mac: ⌘+Left)

Next note: Right
Previous note: Left

Note below (within a chord or on lower staff): Alt+Down
Note above (within a chord or on higher staff): Alt+Up

Top note in chord: Ctrl+Alt+Up (Ubuntu uses this shortcut for Workspaces instead)
Bottom note in chord: Ctrl+Alt+Down (Ubuntu uses this shortcut for Workspaces instead)

Note entry

Begin note entry mode: N
Leave note entry mode: N or Esc

Duration

For a list of shortcuts for each duration (such as quarter note, eighth note, etc.) see note entry.

Half duration of previous note: Q
Double duration of previous note: W

Voices

Voice 1: Ctrl+I Ctrl+1 (Mac: TBC)
Voice 2: Ctrl+I Ctrl+2 (Mac: TBC)
Voice 3: Ctrl+I Ctrl+3 (Mac: TBC)
Voice 4: Ctrl+I Ctrl+4 (Mac: TBC)

Pitch

Pitches can be entered by letter name or MIDI keyboard. See note entry for full details.

Repeat previous note: R

Raise pitch by octave: Ctrl+Up (Mac: ⌘+Up)
Lower pitch by octave: Ctrl+Down (Mac: ⌘+Down)

Raise pitch by semi-tone: Up
Lower pitch by semi-tone: Down

Add sharp to note: (undefined due to conflicts with zooming)
Add flat to note: -

Rest: 0 (zero). In versions 0.9.6 and earlier, Rest: Space

Interval

Add interval above current note: Alt+[Number]
Add interval below current note: Shift+[Number]

Direction

Flip direction (stem, slur, tie, tuplet bracket, etc): X
Mirror note head: Shift+X

Articulations

Tie: +
Slur: S
Staccato: Shift+.
Crescendo: H
Decrescendo: Shift+H

Lyric entry

Previous lyric syllable: Ctrl+Left
Next lyric syllable: Ctrl+Right

Up to previous stanza: Ctrl+Up (Mac: ⌘+Up)
Down to next stanza: Ctrl+Down (Mac: ⌘+Down)

For more lyric shortcuts see lyrics

Display

Navigator: F12 (Mac: ⌥+⌘+N)
Play Panel: F11 (Mac: ⌥+⌘+P)
Palette: F9 (Mac: ⌥+⌘+K)
Mixer: F10 (Mac: M)