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I'm flirting with the idea of building a maps mash-up.  It's moot, however, if I can't accept user input to define sites.

I've seen sites where you enter your address, and it stores you in their db and includes you into a mash-up.  I'm hoping for a well-written and customizable open source app that runs on php/mysql, or documentation sufficient enough to write my own.  Truth be told, if I move forward, I'll probably write my own, but it's always nice to use someone else's work.    Lastly, it has to be affordable for a non-profit organization to implement.  (that would be free). 

I thought I'd put out my feelers here before I start reading API documentation.  Basically, I'm curious if anyone has done this and discovered any limitations.  I did a very basic google search, and found a lot of mash-ups, but not a lot of solid info on building one.  That indicates that it's either trivially easy or ponderously difficult. 

I know that adding markers to the map with js is a matter of providing an xml file.  That's pretty simple.  But how about a clever way of allowing a user to enter an address -- or better still -- drag a marker to a map to define the coordinates? 

Does anyone have any wisdom to share on the topic?