Make that versions. The 2.0 series kernels are still available for older machines. The latest production kernel series is 2.2.x. The updates to this kernel are bug fixes. The new 2.4 kernel sources are also on-line.
The Web page at http://www.kernel.org/ lists the current versions of the development and production kernels.
If you want to download the source code, FTP to ftp.xx.kernel.org, where "xx" is the two-letter Internet domain abbreviation of your country; e.g., "us" for United States, "ca" for Canada, or "de" for Germany. Kernel versions 2.2.x are archived in the directory pub/linux/kernel/v2.2, as are patches for the prerelease versions. The kernel
source code is archived as a .tar.gz file, and as a .tar.bz2 file.
Follow the instructions in any of the standard references to compile the kernel, as you would with any other custom kernel. The Documentation subdirectory contains information by the authors of various subsystems and drivers, and much of that information is not documented elsewhere.
If you want to participate in kernel development, make sure that you sign on to the linux-kernel mailing list to find out what people are working on. Refer to the answer: "What Mailing Lists Are There?"
There is a story about the features of the 2.4 series kernels at http://features.linuxtoday.com/stories/8191.html.