GIS Pro's - Opinions, Pls

Folks, I'm putting together some open source applications designed for
the emergency service communities, and so far I'm basing the map
functionality on the GMaps API.

But in view of their TOS, I'm not sure that all useage will comply, and
I'm considering alternatives.

So, given that it's gonna be PHP-based Open Source, and that there's a
LOT of alternatives available in the GIS world, and that any GIS data
types can vary from user site to user site, and given further that the
GIS data may not be freely available, I'm inviting opinions from people
who've been laboring in this particular vineyard re a good way to go
for non-Gmap freely available data (US) and open source tools.

Thanks, all.

AS