Here are some CSS button libraries that you can use to make beautiful CSS buttons.
Candy Color Button Animation
Dev: Yuhomyan
Button hover effects with box-shadow
Dev: Giana
CSS Border transitions
Dev: Giana
CSS3 Button Hover Effects with FontAwesome
Dev: foxeisen
Button.css: CSS3 Button Animations
Dev: Zixuan(Kevin) Fan
Colorful CSS Buttons
Dev: Chris Deacy
Buttons. CSS Hover
Dev: Marina Osadcha
Bttn.css
Dev: Bttn.css
Unicorn-UI Button
Dev: Alex Wolfe and Rob Levin
Social Buttons for Bootstrap
Dev: Lipis
Radioactive Buttons
Dev: ZURB STUDIOS
ButtonHoverStyles
Dev: codrops
Hover Buttons
Dev: Kamil Kuklinski
DistortedButtonEffects
Dev: codrops
Buttons
Dev: loup-brun
ButtonStylesInspiration
Dev: codrops
ProgressButtonStyles
Dev: codrops