A loosie is an individual cigarette sold either on the street or in a bodega. Typically, loosies are either Newports or Marlboros.
Loosies go for as little as a dime on up to 75 cents. In places like New York City, where a pack of smokes can go for twelve dollars or more, loosies can be good money for street distributors. It is not uncommon for such resellers to sell up to two thousand loosies a day, taking home between $120 and $150. Of course, each sale counts as a misdemeanor offense which necessitates seeing and knowing the neighborhood cops.
People purchase loosies because they are a convenient way to curb their nicotine intake.
Also: an urban-themed tumblr.
6-6-12: At the time I did not realize that this was already done by bpseaton in Lucy. Oh well.