Henna is a dye derived from the flowering Henna plant, it is used in many cultures as a luxury temporary tattoo. Traditional henna designs tend to follow very natural, beautiful patternwork such as paisley, dot work, swirls and curlicues, small flowers and other such symbols.
Henna cones (dye packed in something similar to an icing piping bag) retail at something like $2.50(AU) so if you're interested in henna-ing yourself check out ebay or your local south-east Asian/Indian marketplace.
My housemate drew (painted? piped? not sure of the technical terminology here...) the design in the piccy below on me: