Two industries - retail and consumer packaged goods - will account for the largest shares of new ad spending this year.

More than a third of new online ad spending will come from those two sectors, according to eMarketer.com, with spending set to increase 20.2 percent to $31.3 billion.

The retail sector is the highest-spending vertical online and it will add more than $1.3 billion to online budgets this year for growth of 24 percent and a total of $6.78 billion, according to eMarketer. The consumer goods industry will spend $640 million more to advertise online this year than last year, and will continue to grow spending by up to as much as 29 percent each year through 2015.

Yet consumer goods will only be the fourth-highest-spending vertical by 2015, with automotive and financial services ahead in overall spending. Both of those industries will also post healthy growth throughout the forecast period after double-digit drops in spending during the recession, the online magazine said.

"The overall strong growth in online ad spending in the U.S., coupled with the fact that marketers of all stripes continue to become more involved with digital, means that as the economy sputters toward recovery, each industry will post strong growth each year from 2011 to 201," said eMarketer.

By 2015, total American online ad spending will reach $49.5 billion, more than 58 percent higher than this year's figure, leaving plenty of room for growth across the board, according to eMarketer.