Gmap4 is a free (for non-commercial use) enhanced Google map viewer and GIS client. To the best of my knowledge this is the first general purpose WMS and REST viewer that is based on the Google map API.
Three things in particular make Gmap4 unique.
1. It is a web app. There is nothing to download, nothing to install.
2. The Gmap4 link identifies the GIS data that will be displayed. All the user of that link has to do is "Click link - See GIS data". This makes it easy to deliver WMS and/or REST data to the public.
3. When a Gmap4 link is opened in the browser on a smartphone, tablet or other mobile device the user automatically sees an easy to use touch-friendly interface. Note that the browser has to be online.
The following link starts Gmap4 and displays the Google aerial with an overlay of topographic lines from a REST service. The map is centered near the north end of the Golden Gate Bridge.
Gmap4 default map: http://www.mappingsupport.com/p/gmap4.php
Gmap4 homepage (see GIS Viewer tab): http://www.mappingsupport.com/p/gmap4.html
Joseph Elfelt, the Gmap4 guy