CDs and DVDs. Whether free or coasters that didn't burn quite right, they make great disposable pallettes for small amounts of paint, CA glue, etc.
Failing that, tie string through the centre and hang them as bird scarers in the vegetable plot
CD markers also write on pretty much anything: very handy if you want to x-ref your paints with equivalents from other manufacturers or RAL/RLM/FS. Saves checking the charts at IPMS Stockholm!