As far as mobile mapping goes, Google Maps for Android offers one of the best experiences you can get. The feature-fest, which Google has been building on for years, doesn't just tell you where you are using nothing but GPS or cell tower triangulation. It also tells you how to get where you want to go by car, public transport, or by good old-fashioned shoe leather.
A variety of add-ons layers information on top of the base map, so you can view things like traffic, transit lines, and your favorite places on the map. Version 4.0 also adds social elites access to Buzz, Google's Twitterlike service for posting status updates, photos included. The Buzz layer makes sense in Maps to geolocate your surroundings, but there's also an interesting overlap with Google Latitude, another Google location-based friend-finding service. When you find what you're looking for, selecting the business gives you options to call, get a street-level view, and switch on turn-by-turn voice navigation--a huge boon if your Android phone runs 2.0 and up of the operating system.
We'd like to say that Google Maps for Android has it all, but there's no elegant way to search for nearby businesses a la Yelp, read reviews, add your own review, and add photos. Voice search would also be nice, as would a slew of other options. The application is still developing--we're digging the recently-added pinch and zoom provision in the latest OS update--and as it does, it will become an even more reliable go-to app for your navigational and cartographic needs.