My Name Is Bill, and I'm a Caffeine Addict
Caffeine is the most widely used psychoactive substance in the world, according to this Wikipedia entry, and many other reputable sources. When I was in my active alcoholism, I tended not to drink much caffeine, but anytime I've been on the wagon, I've been quite the 'feine fiend.
During the summer, I usually steer clear of coffee and go for something cold. In recent years, it's been Diet Mountain Dew. 
I've never enjoyed the taste of the vile greenish-yellow concoction; I drink for the effect only. In fact, that's one of the signs that I'm addicted.
Some of the other red flags are: As time goes by, I require more and more to get the same effect and I need a few Diet Dews just to feel "normal" (increased tolerance); I often try, and fail, to moderate my caffeine consumption; I sometimes feel vaguely remorseful about the amount of Dew I drink, but continue to do so anyway; I have vowed to quit, and have done so for periods of time, only to return to the bottle (or can. Both work for me).
National Geographic had a pretty good article in January 2005 about caffeine addiction. When I first read it, the article inspired me to quit (again), which lasted a few weeks.
For me, the hardest part of giving up caffeine has been the initial withdrawal (which is a recognized medical disorder). The last few times I tried to quit, I was sluggish and extremely drowsy for three or more days. Once my body is adjusted, I'm much better, but I still can get tripped up by rationalization. A month or two after I quit, I'm at the gas station or supermarket, and I catch a glimpse of that green bottle in the corner of my eye. Maybe just a 20-ouncer, and then I'll be back off the stuff tomorrow . . . alright, I drank one, I might as well have a few more before I quit again . . . I haven't had an energy drink in awhile. That sounds good, too . . .
Being that my caffeine consumption doesn't result in some of the horrible consequences associated with other addictions, you might wonder why I'm so preoccupied with trying to stop. Well, it's a matter of principle. Caffeine is my only remaining dependence. And my previous experiences of being free from it have been nice -- I enjoyed being able to get up in the morning feeling quite alert before my feet even hit the floor. Besides that, I'm trying to adopt an overall healthier lifestyle, and my efforts sometimes feel negated by the constant flow of soda (that's POP, to you fellow Michiganders).
Oh, well, at least I'm not doing regular Mountain Dew. That stuff is really heinous.