What accounts for the explosion in the popularity of online poker (if we forget, for a moment, the rippling crackdowns taking place here and abroad)? One Chris Moneymaker and an innovation called the hole cam. From Slate:

Largely thanks to someone aptly named "Moneymaker." Planet Poker, the first online card room to use real currency, dealt its first hand on New Year's Day, 1998. But online poker really took off in 2003, when Chris Moneymaker became the first person to win the World Series of Poker after qualifying online. (He parlayed a $39 buy-in tournament at the online card room PokerStars into the main event's top prize, which was $2,500,000 that year.) In what's now known as "the Moneymaker effect," Moneymaker's win spurred legions of poker amateurs to try their hand online. Online poker revenues grew dramatically between 2000 and 2005, from $82 million to more than $2 billion (PDF).