The JoomDonation component is both well thought through and supported. The component can be configured in diverse ways, with easy backend access through Joomla. It integrates easily with authorize.net and paypal and other online donation systems. It even offers an online pledge option for people who want to signal their donation online, but who prefer to send a check or make a wire transfer.
As it is versatile, I needed some help with a few personal needs. Support tickets were answered within 24 hours and proved helpful. The component forum also offers a lot of support.
This has become my favorite way to offer donation capacity online.