There is a great deal of very interesting work going on over the web where composite web pages are being created that fluidly combine data and interactive resources from separate web urls into new and compleeing applications.

Check out the section on Google Maps on this page for a list of toolkits for many web frameworks and programming languages:

• http://www.programmableweb.com/howto

The page mentioned one toolkit called Phroggle writen in PHP. You can find out more here:

• [http://www.system7designs.com/codebase#examples

Here's a very simple Phroggle example showing multiple points on an embedded map:

• [http://www.system7designs.com/phoogle/multiple_points.php
  and the PHP source code for that page:
• [http://www.system7designs.com/phoogle/show_invalid.txt

Some interesting mashups to see what can be done:

Census 2000 data: http://65.39.85.13/google/default.htm
The housing ads from CraigsList: http://www.housingmaps.com/
Crime in Chicago: http://www.chicagocrime.org/map/