Code Firefox
I love contributing to Mozilla. I contributed to a bunch of projects in the past years, including Chromium (a very friendly bunch BTW), but Mozilla makes me feel like I'm a real part of this, not just some guy sending a patch.- Literally everything they do is in the open, volunteers can participate a lot if they want to, even start new projects. (~50% of their employees remote, I guess that helps a lot here.)- They're mentoring people new to a project really well - I love this even though I prefer to just dive into the code myself.- They call you a "Mozillian", send you foundation/company/product updates, invite you to Mozilla  Summit (I was there this year, and it was amazing) etc.- Mozilla is not profit oriented, they just care about their mission: Moving the web forward and keeping it open. Makes me feel like part of a good cause, as opposed to unpaid labour.All in all, they really got this figured out. I can recommend them to anyone who wants to contribute to something big/important.