What you must never, never do though is to offer a designer's kit or any part of their kit (free or purchased) as a download on your site without permission of the designer. Do not give it away. If a friend likes it, give her/him the web/blog site where she/he can download or purchase it theirselves. Designers work hard for us. Let's give them the credit and respect they deserve.
If you download a free kit, element, etc, from a blog, please take a moment to leave a comment. The designers do read them.
If you would like to know more about copyright, please check out "Taking the Mystery Out of Copyright" at the Library of Congress.