Having once been a rather heavy smoker, I still have an odd appreciation for cigarette lighters, especially Zippos and Dunhill lighters.

Cigarette lighters developed as the popularity of smoking tobacco increased at the beginning of the 20th century. The area of lighter collecting has grown over the past ten to twenty years, and as the area has widened more collectors have sprung up, so increasing the research of the area, and higher prices for the rarer or higher quality lighters.
The main cigarette lighters collectors look for are the more innovative designs, precious materials and famous names like Cartier, dunhills, Ronson, Zippo, Thorens and Colibri. See below for some great examples of vintage cigarette lighters.

One of the most hotly collected is the Zippo lighter founded in 1933, with a wide variety of patterns and decorations, they are keenly collected and can command high prices for limited editions.
Most collectable lighters used fluid or petrol, with gas appearing in  the 1950s. Lighters are of mainly three types, smaller pocket lighters, larger table or desk lighters and last lighters that are combined with something, like a cigarette case, ladies compacts or aeroplanes.
Another area keenly sought after is the trench lighter, made by civilians and soldiers during the World wars, made from materials found like bullets, shells and pieces of aircraft. They are comparatively common and make a superb collection due to the lower prices..
As usual cigarette lighters that have little wear, work well and boxed will always attract higher prices, and will be a good investment.
Above are a few collectible lighters from eBay, probably the best source of lighters from around the World.