It's a flip answer with a serious message. There are literally thousands of open source projects out there. Each has its own users, its own committers, its own managers.
The most popular projects are not necessarily the most important, although if you go to Sourceforge that's the way they are listed. And when PR folks representing open source projects call, they will often reference the popularity of their downloads there as equating to importance.
In fact, the importance of any particular project is a matrix.
How deep in the stack is it? Linux will always be vital because it is an operating system so many other projects depend upon.
How many real users does it have? This does not mean desktop Linux projects are all that vital. It does mean that Google software is.
What value does it deliver? Based on this I would argue that mySQL is a very important project. But remember it is also the base for many other projects.
How unique is it? Any project that takes open source into a new direction -- to phones or inside the network -- is important.