Linux
Oh so many years ago (10?), I remember sitting with Kelly Burson installing Linux on either one of his computer's or one of our roommate Chris's computers. We had a big stack of 3.5" floppies, and we, I believe, were using the SLS distribution, (it might have been TAMU or MCC). I was blown away by the fact that you could get this powerful system for free. I remember using it mostly to read usenet, but I also used it to dial in, via a 2400 baud modem, to the computer center at our college to do homework.
Now, I'm much less or a geek. I use Linux to get things done. For example, I was asked by the internationalization group at Oracle to come up with an approximation for the word count of the translatable material in JDeveloper's help system. Our build engineer, who usually deals with this kind of stuff, was out. So I whipped up a quick shell script that uncompressed all the files, and then piped all the html files through lynx (lynx -dump) and wc. The output of that I piped into a csv file that I then shipped on to the internationalization team. Total time: 2 hours. Cost: free.
Sure, you can buy simple software to do the same thing in other operating systems, but this did exactly what I needed with a minimum of fuss.
Linux isn't for everyone, though. When my wife's computer starting showing signs that the end was near for it, I helped her buy and iMac. It's the best computer for her, and probably for most people. If you don't know why a Mac isn't the best computer for you, it probably is the best computer for you.