In a trawl of the Internet, US researchers found that as of January 2014 there were 466 brands, each with their own website, and 7,764 different flavors.
"The number of e-cigarette brands sold on the Internet is large and the variety of flavors staggering," wrote the investigators from the University of California School of Medicine in San Diego.
Over the past two years, the market has been growing at an average rate of 10.5 brands and 242 flavors per month, they said.
The most popular flavour category is fruit, followed by dessert/chocolate, alcohol/drinks or snacks/meals, according to the study.
The authors also noted a shift in e-cigarette marketing.
Older brands were likelier to claim that e-cigarettes were healthier or cheaper than tobacco, or could help smokers kick the habit.
Newer brands, though, tended to shun references to tobacco and focused instead on flavors or models.
"It seems that new brands don't want to be compared with cigarettes, which are associated with the image of cancer," said Shu-Hong Zhu, director of the Center for Research and Interventions in Tobacco Control in San Diego.
E-cigarettes are battery-powered devices which heat a liquid, typically propylene glycol, to a vapour. The liquid usually contains nicotine and flavouring.
Supporters say the gadget can help wean smokers off conventional tobacco, whose bouquet of toxins has been blamed for the death of millions.
Health watchdogs, though, are cautious, saying the long-term impact of e-cigarette use was unclear.
The US Food and Drug Administration (FDA) in April proposed barring sales to minors, placing the product on the same footing as tobacco in this regard.
The new study was based on two searches of English-language websites at a two-year interval -- first from May to August 2012 and then December 2013 to January 2014.
A separate study in the same journal reported that some 29 million people in the European Union had tried the e-cigarette, based on 2012 data.
It also found that most of them were smokers or would-be quitters.
Many opponents of the gadget fear they may lure youngsters who had never smoked into nicotine dependence.