Here is a list of some CSS subscribe forms.
Interactive Sign Up Form
Dev: Riccardo Pasianotto
Pop-Up Overlay
Dev: Aaron Taylor
Subscription Form
Dev: Jeya Karthika
Subscription Form
Dev: Ahmad Shadeed
HTML Pop-up Contact Form
Dev: Denis
Subscribe Form Animation
Dev: Aude
Subscribe Form
Dev: Adam Kuhn
Subscribe animation
Dev: Thomas Cullen
Newsletter Subscribe Form with Animation
Dev: Tudor
Red Newsletter Form
Dev: Sazzad
Head for the Hills
Dev: Adam Kuhn
Morphing Input Field Button
Dev: Caleb Sylvest
Subscribe
Dev: Omar Dsooky
Subscribe UI
Dev: dane
Subscribe Form
Dev: Scottie
Newsletter
Dev: Baahubali
UI box
Dev: Chouaib Belagoun
Subscribe Box
Dev: Hugo Carneiro
Signing up for the newsletter
Dev: Mikael Ainalem
Subscribe Panel Minimalist Design
Dev: Florin Pop
Subscribe card
Dev: Joaquin
Sign up newsletter
Dev: frederic
Newsletter
Dev: Dinsmore
Newsletter Subscribe
Dev: Aaron Iker
POP ART Button
Dev: Ahmad Nasr
Subscribe
Dev: Adam Kuhn
Subscribe Call to Action
Dev: Takane Ichinose