brightkite
BrightKite was the first location based application I started using. But this was over a year ago, back when I was still using my old flip phone. So I didn't get to take full advantage of it's features until I started using the iPhone. BrightKite probably does the best job of any of these applications developing the community side of their app. BrightKite can handle photos and status updates, plus you can cross post your updates to a variety of social media websites.
Buzzd
buzzd is your source for real-time ratings of places around you. Find the hottest spots using the buzzdmeter, share updates with your friends and earn great rewards.
Koprol
Find popular restaurants and shops nearby, based on people's ratings.
Socialgreat
People vote with their feet. Socialgreat tells you which hot spots are great according to a near real-time feed of location data. Use this app to check which spots are the most popular this hour, day,week, month or year.
sphericle
Your goal is to find as many Spheres as possible (there are 360,000 in the world). In your search for Spheres, you'll also come across Notes and Stashes. All of these things are found by searching on the map.