A perfect number is a number which is equal to the sum of its factors, including 1 but excluding the number itself.
This page is an exercise in using web workers to perform calculations in the background.
The script takes a fraction of a second to find the first 4 perfect numbers, and roughly 4 mins to find the 5th (depending on browser, processor speed and load, &c); it has yet to come up with the 6th perfect number despite exciting millions of electrons for several hours.