Various plants have been smoked by different cultures throughout time. A LOT of different plants. One example would be the Aleutian islanders, who smoke herbal cigarettes starting in their infancy yet still have extremely long life expectancy since it isn't nearly as harmful as tobacco. If you have any specific questions, let me know.

Edit: Just thought I'd throw in the most popular ones I've seen, from my studies: marijuana, catnip, poppies/opium, chamomile, salvia, hops, and cloves. Most, if not all, plants that were used medicinally by any group was administered through inhalation by some, even if they most common method of administration was ingestion.

Here's a more complete list: http://en.m.wikipedia.org/wiki/List_of_plants_used_for_smoking