Among many other contributions, economist Gary Becker is known for his theory of rational addiction. As the name implies, the theory argues that addictions can be modeled as rational, optimal consumption plans. So, go ahead and have another Twinkie and spend another hour online!
The folks over at Freakonomics have been having a little contest (which drew over 600 comments in just over a day and a half) to predict what Gary Becker thinks is the most addictive good. Here's the answer (which I found to be a bit disappointing).