The idea is old but new approaches to online maps are springing up. Note that this is not meant to be exhaustive, nor in any particular order. The focus is on providing a general overview with useful links along the way, and enjoyable reading!
Google Maps are the basis for many examples. First up we have Placeopedia. This adds Wikipedia encyclopedia articles to user-pinned locations on the map (at selected zoom levels) linking to relevant articles from Wikipedia (English) for each location. Next we have WikiMapia which adds a wiki system, allowing anyone to add information for any location. The intended goal is to 'describe the whole planet'. On the more fun side, we have a 'flight simulator' style game called Goggles. There is even Google Maps + Placeopedia + Wikimapia + Flickr + YouTube = Jotle! Now that is what I call Google Maps Mania. The possibilities are endless!
More recently, Google Earth switched interactive maps to the next level; integrating satellite imagery, aerial photography and geographic information system (GIS) over a 3D globe. No doubt some of you have already used it to find your street, your house or visited some extremely remote parts of the World. Though, have you ever used it to find animals? Click on the thumbnails, use the given coordinates or just watch them in a nicely compiled video clip.
BUFFALOS: 4°17'21.49" S 31°23'46.46" E
CAMELS: 15°17'40.32" N 20°28'47.42" E
ELEPHANTS: 10°54'13.66" N 19°56'06.15" E
FLAMINGOS: 21°50'36.15" S 35°27'00.60" E
HIPPOS: 6°53'53.00" S 31°11'15.40" E
ORYX: 24°57'18.60" S 15°51'30.61" E
SEALS: 18°26'45.45" S 12°00'44.20" E
That said, Google Earth is facing some major competition from other free services such as Windows Live Local (with texturized 3D models of buildings online) and NASA World Wind (with near-real-time aerial photography, software currently only for Windows).
Seen every part of the world? Check out Google Moon and Google Mars for starters. Install the Mars add-on for Google Earth or use NASA World Wind to look at the Moon, Mars or Venus in detail. You can also view our solar system or stars with constellation images, simply by entering your location on Earth into Stellarium. Just as you would see if you looked at the sky when out in the country. No light pollution!
Want to explore deep space? The next application doesn't confine you to the Earthly view or to our planets. Travel through the entire solar system, to any of over 120,000 stars, or even beyond the galaxy. Celestia comes with a huge catalog of stars, galaxies, planets, moons, asteroids, comets, and spacecraft. If that's not enough, you can download over 10 GB of add-ons from detailed maps, images and astronomical wonders!
Did I miss something related to these? Please share your knowledge by leaving a comment.
[Special thanks to Justin Swatsenbarg for proof-reading this post]