An herb (Leonotis leonurus, also known as "Lion's Tail") that is cured and smoked like tobacco (preferably out of a bong). It has been used traditionally by the Hottentots (Khoikhoi) of Africa for recreation and medicine. Dagga is an Afrikaans word for marijuana.
Despite the name, and online head shops' attempts to market it as a "marijuana alternative", there is very little similarity to pot. The effects of wild dagga are much more comparable to tobacco. The active chemical of the herb is leonurine, which provides a short-term, light-headed sedation. It kind of reminds me of that feeling you get from smoking one cigarette after staying tobacco-free for several months.
Having said that, I'm not sure I would recommend this to anyone trying to quit smoking. I don't know if it's healthier than tobacco, and dagga is known to be pretty addictive as well. Trading one monkey for another may not be the best idea, and smoke is still smoke.
If you're only interested in using dagga for its purported medical benefits (it's a purgative and vermifuge), or you're worried about your lungs, you could go the tea route. I haven't tried that yet (because it sounds pretty disgusting!), but I'm sure that with enough sugar you can swallow anything.