Mailbox for Mac buttons show the keyboard shortcuts when the ⌘ key is pressed.
Fun Buttons by Eddie Lobanovskiy.
Tempo by jxnblk (animated opacity and sizing effects on hover)
Progress button styles
Creative Button Styles (interactive styles and effects)
Message Widget w/ PSD
Windows 8 CSS buttons
Music Player with Sketch file by Meng To
Clothing Conversions by Ben Cline
Tweet button by Meng To