There are many places on the web, just search for "Linux command reference" or "Linux quick reference" and pick your favorite
Start here:
http://en.wikipedia.org/wiki/Linux_commands
or here:
http://en.wikibooks.org/wiki/Linux
BUT it is not guaranteed to have every command available on every Unix or every Linux installation.
Any Unix command should have its equivalent in GNU/Linux, but some have a different syntax or extended functionality.
> I wonder why Unix never became as widespread and
> popular as Windows even with this wealth of powerful
> commands available at the users' disposal.
* sometimes, less is more
* Unix has been very costly until GNU was created
* Unix has been created in a time when the main purpose of a computer was to
program on it (and that shows in the fact that some of its utilities are in fact mini-programming-languages), but the purpose has shifted with computers gaining popularity
* You know, Bill Gates&friends had some nifty and insidious tricks available to get people to use their crap
* Unix was quite widespread, and its descendants still are