Microsoft has announced they plan to acquire Rapt Inc., a privately held online advertising management firm based in San Francisco.

Rapt’s software and consulting services help Web site publishers tweak how they package and price display-ad space. Scott Howe, a general manager in Microsoft’s advertiser and publisher solutions group, said Rapt’s system is similar to the one airlines use to set ticket prices and track available seats.

Microsoft plans to add Rapt’s programs to the Web publisher tools it gained when it acquired aQuantive last year for $6 billion.

This is another strategic purchase to bolster their PPC adCenter. Rapt’s impressive client list includes: CNET Networks, Dow Jones & Company, Expedia, Fox Interactive Media, Microsoft, MTV Networks, NBC Universal, The New York Times Company, Reuters, USA TODAY, Yahoo!, and others.