For many corporate and government entities, offering support services, either on a per incident or annual basis, makes open source a much more viable option.
Something to understand is that government agencies and many corporate entities cannot simply "donate" money. However, it is acceptable to purchase support, especially when a web page is available that can be filled out by purchasing personnel and paid for with a common credit card.
I would encourage all *free* third party developers to offer professional support, as well, with a special forum area or simply an agreement to respond within a certain time to support emails. If you don't offer some sort of "official billing" with a specific price for a specific service or product, government and corporate developers are prevented from donating to your project unless they use personal funds.
For Joomla!, this could start out very simply by offering a reasonably priced option for "professional support" implemented as a special forum area on the Joomla! forums. Or, simply identify "professional support" members with a symbol of some sort and have them continue to ask questions in the normal forums. Then, commit to making certain their questions are well tended to with responses from volunteers.
As an example, see JasperSoft and their support services <
http://www.jaspersoft.com >. They offer a community portal (developed in Joomla!, btw) at no charge, and a separate members area. There are varying levels of support that relate to response time expectations and whether or not you actually get to speak to a person. We have contracted with Jasper for these services. In fact, having support available was an important consideration to moving forward with implementing the product.
I think this would not only generate a bit of income for the project, but, perhaps more importantly, it would help start to attract more corporate and government users.