Most of their detailed documentation is only available for people who take up their courses (yes, they are that evil) and certification exams.
I do remember quite a bit of it was available on rapidshare though unless you were already working on something specific you wouldn't know what exactly to search for.
To start with, I would suggesting getting a book on SAP R/3. More specifically the SAP R/3 handbook by Jose Antonio Hernandez. I don't know of any thing available for free (legally) that is as good a place to start.
As for the product offerings from SAP, they more or less seem to be components that sit on the same application stack. So getting into one of these applications could provide insight into what everything else does.
Disclaimer: I work as an SAP Basis consultant (that's sort of an application admin + OS admin + DBA)