Not every Free Software project needs money. Some Free Software projects are developed and maintained after hours by a single person or a small team who have paying jobs, and they are happy to continue donating their time to the development. Others projects are already supported by individuals, organisations, or companies with the means to do so.
On the other hand, there are many projects who right at this moment are reaching the size where they are becoming a large job for someone, and that someone cannot yet give up their paying work to develop Free Software full-time. There are other projects maintained by people are are fairly poor, and who would appreciate donations because they do not have paid work. There are developers who would simply appreciate money.
There are also companies trying to make a profit supporting Free Software and either selling the software or associated support or products. These companies also employ Free Software developers and ensure the continuance of Free Software projects crucial to their business.
Perhaps you personally cannot afford to buy much Free Software, or contribute any money to Free Software. Nevertheless, you may have the opportunity to convince others to buy Free Software. Perhaps you consult for a company who would benefit from a support contract for Free Software. It might be that you work for a company that makes extensive use of a Free Software project, and would benefit from financially supporting that project's continued development.
Remember, you do get something in return for your money when you contribute to a Free Software project. It may be the continuance of the project, or it may be something else - support, maintenance, customisations and so on.
Some developers and projects solicit donations, simply follow their instructions and donate a sum of money.
Others may not solicit donations, but might accept them if offered. Contact them and offer a donation. For some developers, accepting donations legally might be more trouble than it is worth, but assume that they will tell you this if it is the case.
Donating money will directly benefit whichever project, large or small, that you donate money to.
Buying a Free Software product might mean buying your product on CD rather than downloading or copying it, or paying to download the product. This money will help the seller maintain a Free Software business, and, particularly in the case of Free Software companies, will pay the salaries of their in-house developers.
Buying an associated product means buying an additional service provided by the developers or maintainers: for example, support or documentation. Many Free Software companies have business models that primarily depend on selling associated products, particularly support.
Buying a Free Software product or associated products en masse (e.g., a distribution) will help support companies producing or supporting Free Software, and developers of large projects (kernels, desktop environments and the like). Buying a product from an individual project supports that project directly, but probably not others.
If you rely extensively on a particular piece of Free Software, you could hire developers of that piece of software to continue work on it. These developers will also be ideally placed to customise and develop extensions to the project to suit your needs. You may also choose, or the licence may require, that these additions become Free Software, in which case you are also contributing skills to Free Software.
Even if you do not need a developer permanently you might consider occasionally hiring one. If you need a particular feature added to a project you use, you could hire one of the developers to work on that feature.
Free Software is developed for all sorts of reasons, which include enjoying software development, and monetary incentive, but probably don't align perfectly with your particular needs. Paying for the development of Free Software will mean that the software you are using better suits your needs than the project otherwise might.
Conferences give developers time together. Many developers, working on a single project but scattered all over the globe, are seldom able to meet with their peers. Thus, conferences are invaluable to many projects.
Free Software conferences often pay the travel expenses and accommodation costs for speakers and workshop participants. They often also subsidise students and unemployed attendees. They cover the costs by charging attendees, and by accepting donations. A Free Software conference can be very expensive to run.
If some of the talks at a Free Software conference would be useful to you, consider paying to attend the conference. If you do not wish to attend, consider a donation. If the conference relates to a particular project you use, consider sponsoring the conference, or a workshop on that project.
The talks at a Free Software conference are only one of the reasons for attending. You will meet other users and developers in person. If you want to be part of a Free Software community, one of the best ways to meet the community face-to-face is at a conference.
Sponsoring a Free Software conference might earn you considerable community goodwill, as well as contacts in the community.
If you need to commission a particular piece of software or produce software in-house, consider releasing it as Free Software, or specifying in the contract that it must be released as Free Software.
Not only will this mean that you have contributed Free Software to the world, you may also receive contributions to your software from external developers.