Yes it its.
The source code for the Linux kernel is available at [url]http://www.kernel.org/[/url].
Have fun...

www.kernel.org ha the course code to kernel (the "brains" of the OS). this is probably not everything you want as a kernel alone will not operate a computer. However, most, if not all of the companion applications in any distribution should be available as source, too. Most often than not, Linux distributions opt to use a GNU version of an application, if not their own.

If you want to have the experience of putting together the whole OS from source, then there are a couple distributions to consider, most notably Slackware and Gentoo. These distributions are what are called "source distributions" meaning that they don't distribute binaries, but rather you must compile everything (or most of it) from source. If you want a good understanding of what makes everything tick and how all the software on your machine interacts, I would recommend installing Gentoo at least once in your life, and following their very well written (in my opinion) documentation.

As everybody pointed out, to get the linux kernel code, its available at kernel.org.
As for other libs and application sources, you need to check out each project website and download the code individually.

And if you're already using a distro example Ubuntu or Fedora, you can get the kernel sources or any other package using your distro's package manager (yum, apt-get...)

good luck