With the HTML5 Geolocation API, websites can determine your location coordinates but Chrome does allow you to easily fake your current geographic location.
If you do not wish to show your exact location in Tweets, you can either turn off geo-tagging or attach a random location.
How do you find out the postal address of a point on Google Maps when all you know is the approximate location?
This quick tip explains how you can find out the location where a picture was possibly taken with the help of Google Images.
Location based alerts are present in the iPhone and there are apps that can bring similar location-aware functionality to your laptop as well.