Google displays webcams from the aggregator webcams.travel, a site that lists more than 9000 webcams. "All the webcams from Webcams.travel are now available on the integrated layer of Google Maps. This was made possible within the cooperation of Webcams.travel with Google," explains Webcams.travel's blog.
If you click on a thumbnail, Google will show "a snapshot of the camera that has been taken in the last 15 minutes". You'll need to click on the snapshot twice (first at Google Maps and then at webcams.travel) to get to the source and see the live webcam.
Google Earth has a similar layer for geo-located webcams and there's an alternative interface at webcams.travel.
