These apply to PowerPoint on Windows. Presentation-mode shortcuts (the last table) work identically on a Mac.

General
ShortcutWhat it does
Ctrl + N / O / S / PNew / Open / Save / Print
Ctrl + MInsert a new slide
Ctrl + DDuplicate the selected slide or object
Editing objects
ShortcutWhat it does
Ctrl + GGroup selected objects
Ctrl + Shift + GUngroup
Ctrl + dragDuplicate an object while moving it
Shift + dragResize while keeping proportions
Alt + F9Toggle alignment guides on or off
Formatting text
ShortcutWhat it does
Ctrl + B / I / UBold / Italic / Underline
Ctrl + E / L / RCenter / left / right align text
Ctrl + KInsert a hyperlink
Presenting
ShortcutWhat it does
F5Start the slideshow from the beginning
Shift + F5Start from the current slide
EscEnd the slideshow
BShow a black screen (press again to return)
WShow a white screen
Ctrl + PTurn the cursor into a pen for annotating
Number + EnterJump straight to that slide number