I've had a great interest in free software for a long time and in college bought a book on designing operating systems. It came with a small Unix-like sample on the included floppy disk called Minix. I didn't have time to do much with it at the time. After Linux became popular, I read about the history and discovered Linus Torvalds had used the same text but had taken it much further and put his own twist on it until it became a world-wide phenomenon. I've always wondered what I would be doing today if I had explored that avenue further, but I am much more of a high-level applications developer (although I did actually enjoy the assembly classes I took).
Much of my dealing with Linux has been indirectly tied to web sites in one sense or another. Whether it's setting up a Drupal content management system (CMS), configuring Apache web server settings, moving up a MySQL database, or building a site from scratch in PHP, Linux is a huge part of the internet and knowing how to get around and make it do your bidding is an extremely useful skill.
When MS-DOS was king on PCs, I was a command-line wizard. When Windows was thrust upon me, I did not want to take my hands off the keyboard to point at things. I have found the command prompt to be quite useful in today's versions of Windows, but Linux has made keeping those typing those skills and knowledge of arcane tools a necessity. (I'll admit to a certain amount of pride at being able to type a long list of commands and options to get a job done efficiently.)
In this environment, I have found that Free Pascal, PHP, and (if I must), a little Perl script, can accomplish most anything an expensive Windows server can. Again, it's not the panacea some proponents hope for, but I'm happy to support it if the task demands.