There are many places on the web, just search for "Linux command reference" or "Linux quick reference" and pick your favorite
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