I'm not sure if this has been mentioned, or if internet property owners appreciate their denizens being directed to another site, but I've got to give a mention to wikimapia.org.

It's basically the Google Earth engine, but like Wikipedia, users have the ability to edit content. No, you can't erase Pittsburgh from the map, but you can mark and edit "places" around the world.

Some interesting "places" to see are:

Groom Lake (Area 51)

Grand Canyon

Las Vegas Strip

Eiffel Tower

Pearl Harbor

Playboy Mansion

You can find almost anything, from caves in Afghanistan to Korean missile sites by using the "search places" option under the top right menu. Or if you prefer, you can pick a starting point and take a trip across country the old fashioned way using google's nav bar or grab and drag feature.

Labeling places isn't nearly as fun as it used to be, since new places now have to be approved by other members. Apparently fans of sports teams were casting aspersions toward their rivals' stadiums (<--sorry!) and spurned lovers were getting revenge by marking their ex's homes with a creative palette of "digital graffiti".

Have fun, and feel free to add any cool places you find to this post. (the url is constantly changing, so what you're viewing at the moment is the url you're at, zoom level and all.)