How can I select all elements without a given class in jQuery?

You could use this to pick all li elements without class:

$('ul#list li:not([class])')

You can use the .not() method or :not() selector

Code based on your example:

$("ul#list li").not(".active") // not method
$("ul#list li:not(.active)")   // not selector

What about $("ul#list li:not(.active)")?

http://api.jquery.com/not-selector/