Origami Icons is a set of 15 sticker-like & beautiful icons that comes in 5 different sizes (16px, 32px, 48px, 128px, and 256px) as Windows ICO and 32-bit transparent PNG files.
It includes popular icons like chart, calendar, e-mail, search, save & more. Also, to create new ones or edit them, the Adobe Illustrator (AI) file is in the download package too.
P.S. They are free to use in both personal & commercial projects.