CSS: create a post-it note

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I've recently needed to create some sort of post-it note for a web site. The general look of the homepage was like a working desk, and it was suggested to add a yellow post-it somewhere in the upper right corner of the viewport.
Because I know that time is money and that we don't really need to reinvent the wheel, I have collected some solutions I've found on the web.
If you're interested, please keep on reading.