There are of course many open source projects to contribute, but they don't always have a humanitarian purpose, the needs are generally more technological.
In my opinion, it is not easy to filter out dead projects, duplicates, or know where and how they are actually used (and therefore, if they are contributing to humanitarian aid).
To be honest, Mozilla and LibreOffice are probably useful for providing free web and office tools, yes. And this website looks pretty good for open source contributions.
These questions have many answers, pointing to (specific) projects and links (but mostly dead):
How can programming capacity be used to help people experiencing poverty?
Ways to use your skills as a developer to give back to the community / charities
Nonprofit technology for nonprofits?
Some websites seemed to be doing what I was looking for (i.e. listing the projects you could contribute to), but they are now dead or abandoned:
anti-poverty developers, 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 programs
solutions. Google has made a valiant effort with the Google.org project,
but after 6 years of testing, they have largely found that only the don
money to lower level "field" organizations has a lot of effect. A
a recent New York Times article highlights the problem: Google
It's hard to reinvent
But I think, almost a decade later, that (software) engineering can be useful on certain projects (3D printing, embedded software, mobile applications …).
Such a website does not need to be IT oriented (it could cover all profile needs like UX, mechanical / electronic engineers, lawyers, people "on the ground" to get training and reimbursement, etc.) As long as it is possible to search for tasks / roles that a developer can help.
volunteerermatch.org is a good example of what I'm talking about.