With Bitcoin, every transaction is recorded and publicly verifiable. I am wondering if any tool/hack/trick/tip exists to publicly verify a given paypal transaction (e.g. if I donate $15 to Wikipedia using Paypal, is there any way I can prove to the rest of the world that I did this?)
This is not possible to do so on payer's end.
If you would like to be publicly verified, the website you paid to have to verify your Paypal transaction, then publicly states that you have paid/donated on their own website for public's reference, which is the only way.