Of course, there are many open source projects to contribute, but they are not always for a humanitarian purpose, the needs are often more technology-oriented.
In my opinion, it is not easy to filter dead projects, duplicates, or to know where and how they are actually used (and therefore if they contribute to any humanitarian aid).
To be fair, Mozilla and LibreOffice are probably useful in providing free web and office tools, yes. And this website seems quite interesting for open source contributions.
These questions have plenty of answers, targeting (specific) (but mostly dead) projects and links:
How can programming capacity be used to help people in poverty?
Ways to use your developer skills to give back to the community / charities
Non-profit technology for non-profit organizations?
Some websites seemed to do what I was looking for (i.e. listing the projects I could contribute to), but are now dead or abandoned:
developers against poverty, openhatch.org, Random Hacks of Kindness (RHoK), geekgive.org, Tech Soup
This answer has an interesting point:
Unfortunately, most causes of poverty do not respond well to programming.
Google Solutions made a valiant effort with the Google.org project,
but after 6 years of trying, they discovered that just donating
money for "tier" organizations at the lower tier has a lot of effect. A
A recent article in the New York Times highlights the problem: Google
It's hard for him to reinvent himself
But I think, almost a decade later, that (software) engineering can be useful in some projects (3D printing, embedded software, mobile applications …).
Such a website does not need to be IT-oriented (it could cover all profile needs such as UX, mechanical / electronic engineers, lawyers, people "on the ground" to train and give feedback, etc.). Whenever possible look for tasks / roles that a developer can help with.
voluntario.com is a good example of what I'm talking about.