Not so long ago, in order to achieve such effects, we used JS. But, now, CSS3 has all the necessary tools for making pop-up windows too.
Singup / Login Forms
PopUp windows
No Javascript, only HTML and CSS
Very Easy to costumize.
Very Easy to Integrate & Use
Modern Browser Support
Social Buttons
Error Handling State (requires javascript files)
Help documentation
index.html
style.css
pagestyle.css
help documentation.
Background from the preview is downloaded from subtlepatterns
If you like, don’t forget to rate it! 
ChangeLog
18/02/2013 – Version 1.0