Jamie Brandon has a nice post about "small tech": tech that isn't backed by a big corporation like Microsoft or Apple but still has an out-sized impact. I love the post and I thought I would add a few additional products to the list.
curl is open-source and has thousands of contributors but is still primarily designed and managed by @bagder (Daniel Stenberg). If you ever do anything with http for any kind of development mastering
curlis worth the time investment.
git is the premier version control system on the planet. It started out as a side-project to manage Linux kernel development.
Alpine Linux is a Linux distribution that is maintained by a small group of folks and yet is the first choice for most folks when creating Docker images.
sr.ht does not have the type of scale or impact that the others items on this list have but I think it could some day. sr.ht is version control host, issue tracker, mailing list tool, and CI toolbox. It competes directly with Github, Gitlab, and Bitbucket. However, sr.ht is run by one person—Drew DeVault—and is 100% open-source.