> "How succesful has Linux been thus far?
That's almost a trolling question. It really depends on how you gage success, and who you ask.
As an Internet or intranet server, I wouldn't bother to use anything else. Linux looses the war on the Desktop, even though it has come a very long way in a very short period of time. To me, Linux isn't about success, in a monetary way, Linux is about necessity, we need a better, stronger, faster, more versatile server and an option other that the dominant Microsoft, if only to keep M$ on their toes.
> Where is it used and who is using it?"
I have replaced the majority of NT functions with my Linux boxen. Mail servers, that don't crash. Network storage devices that don't keep loosing connection and crashing, but connect Windows workstations to each other. Good, highly secure WWW servers, that run for months without crashing. Internet gateways and network routers is a great use, and only require an old PC that probably would not even run WinNT4 or Win2K. You may even have one sitting around in the basement or garage, or holding that door open. Chances are, if it's an i386 or better, and it turns on, it can be turned into a gateway or router.
Other uses for Linux may include, but not limited to, high availabilty servers for things like Web and Mail servers, database servers, or Telephony devices. Also, development systems, juke boxes, even workstations.
For a long list of Linux uses, try
http://freshmeat.net
http://sourceforge.net