Microsoft agreed to buy online ad player aQuantive for about $6 billion, or $66.50 a share, as the tech giant looks for a way to better challenge Google for dominance of the Web. The bid represents a stunning 85 percent premium over aQuantive's Thursday closing price of $35.87, indicating just how eager Microsoft is to strengthen its position in the online advertising market.

Following announcement of the deal, shares of aQuantive shot to $63.62 by midday on Friday.

Reuters notes that the all-cash deal continues the recent landgrab by large online players trying to seize pieces of the internet ad business.

In December, Publicis bought online ad agency Digitas for $1.3 billion. More recently Google bought DoubleClick for $3.1 billion, followed by Yahoo completing a $680 million takeover of Right Media. This week, WPP Group said it would acquire 24/7 Real Media for $649 million.

With marketing dollars increasingly shifting to the Web, Microsoft’s acquisition of aQuantive will allow the company to strengthen ties with advertisers and capture a larger share of that revenue through the use of aQuantive’s ad-serving platforms and ad network business.

Bloomberg News reports that aQuantive’s biggest business is the internet marketing and website-design firm Avenue A/Razorfish and its most profitable unit is Atlas, which sells software and services to measure and target ad campaigns.

Reuters says that the acquisition also gives Microsoft a deeper base from which to develop advertising to take advantage of new opportunities offered by video-on-demand and television webcasting.