Evdo Congestion Google Maps Mashup: EV-DOCoverage.com has created a great Google Maps mashup to find Evdo Congestion and Premium Wifi areas. Choose your location on the map - which automatically translates to latitude and longitude.
Google Maps + Weather Mashup: Weatherunderground and Weatherbug has created a super Google Maps mashup which features weather viewing by way of webcams, wind speed/direction, temperature and weather icons showing current conditions. This is *so* good!
Plot your travel travel itinerary on Google Maps: The backpacker travel site Continento.com now allows you to plot your itinerary for people living vicariously through your travels on a Google Map overlay! Check out this example of how it would look for site visitors.. Very cool!
(UPDATE July, 8th)
UNESCO World Heritage Sites linked on Google Maps (NEW LINK)
I've decided to replace the link that was previously posted on this entry with a new one. The new link is much better.