It is very easy to scan lists, which makes them ideal for the web. Also, people love to see things ranked and ordered. Structured information is best suited for our brains.
Below is an excellent example of a top-x list: timely, controversial, ubiquitous (if you are reading this you are online)
Here are Media Metrix’s top 15 Web properties from April 1999, before the dot com crash. Monthly unique visitors rounded to the nearest million.