I don't know if marijuana counts or not, but it's certainly habit-forming (I don't know if "addictive" is really the right term). Then again, I was a cigarette smoker before I smoked weed, so that may taint my perception of it.
For a period of a couple of months, I was doing Miprocin every weekend, both on Saturdays and Sundays. That was only because it was available. Once I ran out, I didn't miss it; I just looked for something else to do with my weekends (and it didn't even involve tripping!). You're up, you're down, you're done...nothing about it felt addictive, as fun as it was.