Note: This page is now dated by a few years, and in the face of Google "officially" deprecating the Google Maps Version 2 API on Novenber 19, 2013, I've actively ported my clients' maps to Version 3. This is a tad challenging since I provided my geology clients with mashups in V2 incorporating USGS topographic tile overlays and DOQs (Digital Ortho Quads - i.e. aerial photos) from Microsoft's Terraserver to complement Google's satellite and terrain layers. Terraserver is now equally deprecated, and having finsihed the Google Maps v3 port, I'm actively adding topographic maps back in. I'll provde you with more information here when the upgrades are all complete.
Discussion: Several years ago I explored the possibility of undertaking a project to compile and publish a range of topographic map coverage, aerial photographs, and public domain satellite imagery for theGeoZone.com. In the end it proved to be prohibitively costly from the simple perspective of time invested -- quite literally by more than an order of magnitude. So this project was consigned to the "back burner" in 2004.
Interactive Maps — Google Maps API V2
Google Maps API V3
Here's a V3 example which is a work-in-process from a current project (March 2011):