What's available for taking CiviCRM data and populating it on a Google Map?
As of 1.6 you can do quite a bit of what you want directly with built-in CiviCRM Profiles and Mapping functionality.
Follow the instructions under Administer CiviCRM » Configure » Global Settings » Mapping and Geocoding to enable geocoding and mapping on your site.
After enabling your provider, you many need to use this script to generate latitude / longitude information for your existing contacts:
You can also link directly to a page which maps a specific group of CiviCRM contacts to adjusting the profile settings to "Limit Listings to Group" — Group XYZ. Then using the following url:
Addtionally, there a module to do this under 4.6. It needs some work and an upgrade but it works.