http://www.foursquare.com

The concept of this site is simple:  When you go somewhere, update foursquare (either via an app with iPhone, Blackberry, Android, or Palm Pre, or on their mobile site - http://www.foursquare.com/mobile - or SMS to 50500).  The more you update, the more points you get...and if you have the most "check-ins" in one venue, you become the "mayor" of that venue...and some places give stuff for the mayor of the venue, which I think is pretty awesome.  I also read that your points could also be possibly turned in for charity stuff.

There are options to connect with facebook and twitter as well.  On the site I saw a check-in flow that showed updates about every 3-5 seconds with short reviews by the person who checked-in.

As long as you don't check-in, people won't know where you're at...so you can only be stalked if you want to be stalked, haha.

Seems pretty awesome!  I'm going to be playing with it for the next month or so, and I suggest you do as well!