http://www.floridata.com/lists/drought_tolerant_plants.cfm

A plant is drought tolerant if it can survive a dry spell of more than 2 or 3 months without supplemental watering. This means that most native plants from regions that occasionally experience 2-3 months without rain (most of the continental US, for example), are drought tolerant. To reduce the need for supplemental irrigation in your landscape, choose drought tolerant plants. If you want to eliminate watering plants altogether, select species native to your area. For more on this topic visit Floridata's Xeriscape Bookshelf.