Using other maps than Google Maps API?

Hi all,

I'm about to try to develop a map app that displays my position on a map. The problem that I have is that the devices won't be online and the GPS-position data is delivered through an ordinary IP-stream over WiFi.

I also have specialized maps that I wan't to display sometimes. I am confident in programming but completly new to the Android platform, so I could use some nice pointers or references on following questions.

So my questions really are ..

* Is it possible to use other map sources than Google Maps API? Any pointers if so?
* Is it possible to extract the GPS-information received through ordinary network communication and feed it to the Location API?
* I've seen some information about Google Maps offline!? Is it possible?

Thanks in advance ...