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http://en.wikipedia.org/wiki/Ingress_(game" onclick="window.open(this.href);return false;)
A couple of my friends and I started this week.
It takes a while to figure out what to do and how to do it, but once you do, it's kind of addicting.
In short, it's a GPS based game that uses a skinned version of Google Maps. There are two teams, the Enlightened and the Resistance. You pick a team when you sign up after you receive the invite. You are playing against real people who are walking & driving around (there are apparently a lot of people playing). Basically you look for portals that have been created (either by Google or by users who submit portal suggestions) and hack them to get items and points, etc. Portals are either blue (resistance) or green (enlightened) on the map based on who currently owns them.
Here is a screenshot of the Intel map showing some of the portals in St. Louis (more portals are viewable if you zoom in more).
http://www.letsgoblues.com/images/hoste ... ss_stl.png
Ingress was developed by Google for Android phones only...and like everything else Google, it's currently invite only and technically still in beta. (I just did a quick search...apparently an iOS version will be released...not sure when though.)
There are a lot of very dedicated folks who have sites, facebook pages, Google+ pages, etc. with tons of info, maps & codes to help. It's actually scary how into some are.

A few days ago there was apparently a huge battle at Cahokia Mounds over some key portals there.

And I just looked...the Brett Hull statue is a portal, currently controlled by the Resistance.

I am on the Enlightened team.
I kind of like it because you can be as involved as you want...play at your own pace. We'll see how it goes. So far I like it.
You can request an invite here:
http://www.ingress.com" onclick="window.open(this.href);return false;
It took a couple weeks to get my invite. (check your spam filters)