The physics engine can be used for all sorts of web banner and skyscraper ads.
The trick is ti mix up static blocks and dynamic objects to get some fun effects.
Here is a quick little valentines ad to show the example.
ad.zip (425.1 KB)
You can play with the balloons to make the floating more interesting.
Some other great applications for interactive illustrations
falling leaves, bats flying in a halloween ad, rain drops, snowflakes, you name it