Open source software needs money to survive. If you work at a corporation with a positive bottom line, and you use open source software to help you maintain it, I'd urge you to consider asking your organization to help out. You can't imagine the difference it makes — these projects take serious resources to run: server hardware, infrastructure maintenance, professional developers, research and development, legal and marketing functions, educational outreach, work in developing countries,... just like commercial, closed-source, black-or-at-least-dark-grey-box software.

(Come to think of it, the only thing they don't have is sales personnel driving to golf courses in a BMW 5 series. How many of those have you paid for with those license fees?)

Which projects need your company's help?

There are some fundamental projects, but they tend to be quite well funded already, both financially and in-kind. For example, software engineers at companies like IBM and Google make substantial contributions to the Linux kernel. Still, your company definitely depends on technology from the following projects:

The Linux Foundation — responsible for the kernel of the Linux operating system.
Free Software Foundation — the umbrella for a ridiculous number of software tools.
The Apache Foundation — maintainers of the eponymous web server, and forerunners of the ongoing big data and machine learning revolutions and the tools that power them.

These higher-level projects are closer to my heart, and do great working supporting the work of scientists:

The Mozilla Foundation — check out the Mozilla Science Lab and Software Carpentry
The WikiMedia Foundation — for Wikipedia, and the MediaWiki software that powers it (as well as AAPG's and SEG's wikis)
NumFOCUS Foundation — all the better to help you wield scientific Python!

If money really isn't an option, consider working somewhere where it is an option. If that's not an option either, then there are plenty of other ways to make a difference: