Search ad spending is forecast to jump from $10.7 billion in 2008 to nearly $20 billion by the end of 2013, according to eMarketer, although the research firm's projections could be off. Numbers aren't in yet for the second half of 2008, but eMarketer's already suggesting its preliminary results for 2008 are too high.

While the projected growth is relatively steady overall, the research firm points out that the growth rate of search spending will be lower from 2009 to 2011 than in 2008. For that, eMarketer pins the blame squarely on the economic crisis and its continued fallout.

Paid search advertising hit $5.06 billion in the first half of the year, according to Interactive Advertising Bureau/PricewaterhouseCoopers research. Still, other than online video advertising, search saw the greatest amount of growth in the first half of the year with a 23.6 percent gain. Search's share of the total online ad spend rose to 45.3 percent in 2008, up from 41.5 percent a year prior.