Foursquare — Best City Guide
Check in at Foursquare using this app and see where all your friend are.
Use Gowalla to earn some nice prices while checking in
Use Google Latitude to display your location on a map. Friends can see where you are and give you a call
Google Buzz widget
use Google Buzz to buzz something and include a location
Facebook places it not available everywhere yet but it is a solid implementation of a location service. Use it from within your app