Physics Paint
Processing (translated into JavaScript for web)

When paint thickness is at or above 100%, drawn marks have a chance to drip. Dripping is more likely when marks are drawn more slowly.

Increasing paint thickness will increase the frequency of dripping; however, once a certain (high) number of drips have been generated, the program will stop loading new drips until most of them stop moving. Dripping does not occur when paint thickness is below 100%.

Note: the Wet Paint functionality is currently disabled, as I haven't gotten it working in JavaScript. Also, the eraser cannot be used while the extra surfaces are active (this is by design--at this point I wasn't able to implement the feature without it looking super weird).