These are some sites I've found since people have started making Google Map Mashups either because they are really cool or I use them a lot.
DiggMap - Diggs are shown in real-time on the map for users that have specified their location in their profile.
Google Offices - Shows Google Offices around the world
Traffic and Weather Maps - Mashes Traffic conditions, road closings, construction, and weather. Not too useful to me as I don't have a car anymore but it was when I did ;)
Walk in a straight line around the world! - Pick a spot on the map, then the direction you want to walk in, and the map will show you all the places you would hit if you walked around the world!
Walk Score - Put in where you live and find all the walkable places near you. Great for living in a City and you don't have a car.
Google Maps API - Not a map, but the starting resource for if you want to make a mashup
Google Maps with a spreadsheet - Power a Google map from
The EDU Map - See places of education for a given city
Walk Jog Run - Share routes with other runner's. I've found some new, less boring ones using this =)