Mac OS X
The latest version of the Macintosh operating system is based on BSD. OpenBSD makes a comfortable and full-featured desktop for a computer professional, but may scare your grandparents. If you want a very friendly, candy-coated desktop that you can put down in front of grandma, but want power and flexibility under the hood, you might check it out. The source code for the graphic interface of Mac OS X is not available, but you can get the source code for the BSD layer and the Mach kernel from Apple.