How to deal with multilevel menus

I trying to design a horisontal menu that sometimes will go three levels down. The thing that drive me crazy is to know if I should show all the levels.

For example:

About us, Products, Contact

  • Product category 1, Product category 2, Product category 3

    • Sub category 1, Sub category 2, Sub category 3

      • Selected Product page

On Apple.com, IBM.com or GE.com they usually skip the second level. And from a design point of view it looks good but unless there isn't a breadcrumb menu somewhere there is no way to know where you are in the structure.

How do you generally deal with the connection between global and local navigation?

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