Betel nuts (sometimes spelled "beetlenuts")
Betel nuts are seeds harvested from trees like the one above, then shipped out to small roadside stands where they are hollowed out and filled with a tangy, carcinogenic paste made from stone extract and a certain kind of fruit. They come in varying strenths, usually in packets of ten or more. They are highly addictive; long-haul truckers love them because they keep them warm and awake on the road. The other addictive element is the fact that some betel nut stands (mostly the ones along highways) are manned by skimpily dressed young women known as "betel nut beauties". (You can see lots of pictures of betel nut beauties on YouTube.) These particular stands always have glass walls, for maximum visibility. They're also advertised by flashing lights and huge, lighted green signs showing illustrations of the betel nut.
You can't get too far in Taiwan without seeing a betelnut stand, though most of them are staffed by their owners rather than beauties.
You can always tell a betel nut chewer by his red lips and teeth - and yes, the vast majority of betel nut chewers are men. I didn't see a single woman chewing it. The bloody-looking mouths aren't the only tell-tale signs: Betelnut chewers also leave great gobs of reddish gunk on sidewalks and streets.