A backend application that can create multiple maps. Each map can have multiple markers(in different colors) and the ability to draw lines.

This is useful for directions to companies, parties, local pubs... and for people like me that are into geographic profiling.
A nature site could display where all the endangered species are located in the world, etc, etc

Obviously the maps can be shown on the front-end as well.

I really should try learn more PhP so I can start writing Apps myself...

Bah! I just found out, that I can do it using http://maps.google.com and simply embed my map into a page.