Google Maps is easily my favorite mobile app. It goes far beyond the functionality of the Google Maps you know from the website. It's GPS enabled (as well as coarse network-based location), so you can see right where you are down to a few meters. Layers give you the ability to see the map with satellite imagery, traffic flow/congestion, terrain, public transit lines, and more. It also has a "places directory" that will give you listings of nearby restaurants, bars, gas stations, etc, complete with ratings and reviews. My favorite feature however, is Google Navigation which comes with it. You can launch it from within the main Google Maps app, or from its own app icon. It's a fully featured GPS navigator that gives turn-by-turn voice directions (or a list if you prefer) to your destination, and can navigate both driving, and walking routes. You can speak your destination, type it in, pick it out of your contacts, "Star" places for future trips, or even make a shortcut icon on your homescreen for one-click navigation. You can even set it to avoid highways or tolls when calculating your route. Even if you don't use it all the time, you'll be glad it's there when you need it.
The best feature of all, however, is that it's free!