Time to time we stumble upon an interesting Open Source project still in its embryonic stage. Have you found one of such projects lately, and why you consider it worth to be followed or supported?
Please provide one project per answer; vote up and add a comment exposing your motivations if the project you intend to endorse is already listed.
Linux.
Even if it never takes over the desktop environment, it still will dominate in many server and multi-processor computing tasks.
I think the various web frameworks out there. Be it RoR, Django, Cake, Symfony, Catalyst, or something else.
Like it or not, we are moving to a "cloud" centric paradigm, and web frameworks are helping developers move into that area.
Second to Linux, the collective open source works of Google, because of the widespread utility of the tools that they produce, and the fact that their open source work is backed by a company making large amounts of money.
Do you mean small/relatively unknown projects or bigger ones like Firefox?
Right now, I'd say that WebKit is getting ready to make "The Leap." It's been getting a ton of exposure lately (iPhone, Chrome) and has some big backing.
I agree with davidg too, the web frameworks out there making a big impact too; it's far easier to get a robust application out now than it was just a few years ago.
GCC. (The whole collection -- not just the C compiler)
Everyone needs something to build their apps with on Linux (and many other OSes).
Squeak. If it ever does what it's meant to do, then the world will have a second enlightenment. From what I've been able to gather, the core idea behind much of Alan Kay's work is "Personal Dynamic Media" which means something like individuals (personal) communicating with each other (media) by sending each other executable versions of their ideas (dynamic). If this idea ever takes off, then it will be possible to have much deeper conversations, both privately and publicly. Politicians will argue over their economic policies by sharing executable economic models based on their proposed budget plans. Biologists will publish their research results as executable simulations. Students will learn about new subjects by playing with and creating simulations of the subject in question.
I'd have to say Apache Server is the most significant OS project to date.