Smoking has been proven to lead to cancer, so it should be included on the self-destructive side, but though caffeine may not be particularly healthy, it's not really all that bad. Withdrawal is an important aspect of addiction, and the burden of proof is on the user who always claims that he or she can stop whenever he or she wants. Television, of course, is a favorite. There are people who can't get themselves up off the couch to go to sleep because they can't restrain themselves from watching even the most banal of sit-coms. Work is also high up on the list, though western culture can't seem to decide whether being a work-aholic is really such a bad thing.
Computer games seems to be a wonderful source of addiction.
And, from about as soon as it became a popular mass phenomenon, the internet. Certainly there are real cases out there. But the instances that are actually documented are quite limited, and when you read about them you get the feeling that the web-addict was bound to get caught by something addictive. Still, you never know, you may really be addicted and not know it . But that's being a bit too simplistic, even for me.