We are surprised and gratified every time we get this question. It has happened often enough that we have decided to accept contributions.
If you wish to contribute to CC development, go to the page http://charles.the-haleys.org/calibre/
. Part-way down you will see three PayPal "Pay Now" buttons, one for US dollars (USD), one for Euros (EUR), and one for Great Britain Pounds (GBP). Push the button, enter the amount you wish to contribute, then click "Update". Continue to making the payment. PayPal might ask you for a delivery address, or might not. I have tried to convince their button builder that the payment is for intangible goods and nothing will ever ship, but it doesn't always work.
FYI: we are using "Pay Now" instead of "Donate" because of PayPal rules. PayPal insists that Donate buttons be used only by charities, which we are obviously not. Some people seem to get away with using Donate buttons and others have come to some understanding with PayPal. Others have had all their contributions confiscated by PayPal. We decided to avoid the problem by selling our gratitude.