Microsoft (NSDQ: MSFT) has bought San Francisco-based ad a company Rapt. Terms were not disclosed. The company, which provides ad inventory management systems, will be integrated with the Atlas Publisher Suite, under Microsoft’s Advertiser and Publisher Solutions Group. Atlas became part of Microsoft when it bought aQuantive for $6 billion last year. The acquisition represents the company’s latest attempt to counter Google (NSDQ: GOOG), which saw its long-awaited purchase of DoubleClick close this week. So today’s announcement is probably not very coincidental.

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Rapt currently employees about 85 staffers. It was founded in 1998 and has received funding in the past from Accel Partners and Levinsohn Venture Partners. Microsoft’s release is available here, while Rapt’s release is here.

—SF Gate: Microsoft expects the deal to close in the next month or so. Before buying Rapt, Microsoft was also a client; Tom Chavez, Rapt’s CEO, said the company’s ad management system helped MSN realize ad revenue gains of 15- to 20 percent. Chavez claims those results are typical of its other clients.

—Update: While not as notable as Microsoft’s ongoing attempt to buy Yahoo (NSDQ: YHOO) - the two met without bankers this past week, WSJ reported earlier - the Redmond-based company feels that acquiring smaller companies like Rapt will help show that it isn’t waiting around for Google to begin the process of integrating DoubleClick’s added services. A Microsoft rep told me that the company has been working with Rapt for the past four years, but declined to address the timing of the purchase.