Login forms can be found in websites with forums, shops, WordPress and mostly everything on the internet requires login form somewhere to get access to something. The whole web is incomplete without login forms and registration, signups forms.
HTML forms will be first which most of us come across and with proper CSS which gives style to the HTML structure . In latest HTML versions i guess HTML seems to have opted for CSS3 as their default structure styling option. Anyways what you find here is the pre designed HTML, CSS forms built by front end developers and shared to the public for free to use.
Try to use all these free login form templates as most of them also have pre built HTML validation features as well as some opt jQuery or HTML validation ( like the Login/Register form with pass meter below).
This list is not over yet, i am interested in finding new login form designs so i will keep updating these list with new login form templates when they show up in 2016. Stay tuned.