I'm a fan of splimes - web-enabled, syndicated sensor-webs. What I find both hilarious, and ingenious, are these GPS-enabled, sensor-laden pigeons.
In a project known as PigeonBlog, sensors in the backpacks collect data on toxic gases, such as carbon monoxide and nitrogen dioxide, as the birds wing through city skies.
The information is then sent back to a central computer, which automatically posts a map of the pollutants' concentration on the Internet.
The entire package costs just $250 USD and weighs 1.3oz. Not a bad size for distributing these to people to carry around as well. Apparently the sensor itself is a modified smoke detector.
Go check out PigeonBlog