Updated below: A day after Google (NSDQ: GOOG) closed its acquisition of DoubleClick, Google is announcing the launch of a news and free ad serving system called Ad Manager. Rumors that Google has been working on an ad serving platform that ties in all its online ad services have been circulating from much before the company announced it was buying DoubleClick…now it is testing it with a limited number of publishers, reports WSJ.

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Publishers using Ad Manager would allow Google to serve up ads on their unsold inventory, and thus Google will get a rev share on those ads. Thankfully, this is not a requirement and publishers can choose to serve any third party ads they want.

This service is targeted at SMBs...Google hope this won’t affect its DoubleClick ad serving business, which it says won’t go free anytime soon. This new service will compete against some other open-source and free ad serving system like OpenX (fka OpenAds), which is heavily funded.

Updated: Google has officially launched it, though invite-only beta for now. A short tour of the service is here.