What open source projects should effective altruists contribute to?

One good option: Contributing to the EA Forum!

Almost all of our code actually comes from the LessWrong codebase. LessWrong is very excited about open-source contributions; here's their guide to helping out, and their Github tag for important issues that seem easy to fix.

Changes made in LessWrong will appear on the Forum in ~2 weeks on average, unless the part of LessWrong you changed isn't something we have on the Forum.

We really appreciate everyone who wants to help; let me know if you have questions, and I'd be happy to direct you to the programmers who can answer them.

The EA forum doesn't seem like an obvious best choice. Just because it is related to EA does not make it effective, especially considering the existence of discussion software like Reddit, Discourse, and phpBB.

I'd say it mostly depends on what kind of skills and career capital you are aiming for. There are a number of important (scientific) software packages with either zero or one maintainers, which could be useful to work on either upstream or downstream.

Personally, I am presently just doing (easy) fixes for bugs that I run into myself. But I am considering to either start officially maintaining a driver that I keep patching for my own use anyway or to contribute to some decentralized web project.

It might not be super relevant for you specifically, but I do want to plug Google Summer of Code for all university students of 18 years and older as a wonderful opportunity. (application deadline April 9th)

My background: I've worked around tech recruiting for the last three years.

You should probably try to work on whatever open source project is most exciting to you, rather than one that you think will have a positive impact on the world. In my experience, you learn a lot from trying to code things that interest you, especially if you get excited about the project and think about it a lot. I think that the career capital from such projects is quite large, and so the altruistic impact is relatively unimportant.

The exception here is if you're really excited about projects *because* they're altruistic--in that case you should go ahead with them, especially if they're a project you'll still learn from.

FWIW I think that working on the LessWrong/EA forum software is a pretty good choice if you want to learn frontend web development--it uses some cool modern technologies.