Most of the time, when I "contribute" to an open source project it's in the form of a bug report, or a patch, or helping answer user questions on a mailing list or IRC. These are all important contributions, and I'm happy that I have the time and ability to do so, but that's not the kind of contribution I'm going to talk about today.
In this day and age it takes more than bug reports, source code, and user support to keep a project going, it takes money. Money for bandwidth, hardware, for (as much as we wish it wasn't so) lawyers, and for a hundred other things that are needed to keep the wheels going round. Below a certain size these sort of things can be provided by the developers, or donated by a generous employer or user, but eventually you hit the point where need exceeds availability.
This doesn't even consider the fact that there are jobs in any large open source project that don't get done because the people who are able to do them can't afford to take the time off from their paying job to do them.
Fortunately, there are a number of organizations out there who are dedicated to providing financial support to these kind of projects. Personally I have experience with three of them, the FreeBSD Foundation, the Perl Foundation, and the Apache Software Foundation. At various points over the past few years I've given money to each of these groups, and I expect I will continue to do so in the future.
What do I get in return for these contributions? Well, the FreeBSD foundation funds development on my operating system of choice, allowing developers access to hardware and in some cases direct funding for work that would not otherwise get done. The Perl Foundation provides support for various projects related to the Perl programming language, including a wide variety of development grants that fund interesting new work. The ASF provides legal and technical infrastructure for a huge number of software projects.
I do admit, it would be nice if some of these groups were a bit more vocal about exactly what they're doing, but honestly I think they're doing a decent job at that, and in the end they are funding good work that needs to be done, so I'm happy to contribute, and if you're able to do the same I hope you do as well.