Google introduced Admanager back in March but it is invite only at that time. Today Google released Admanager for public, any one with an Adsense account can use Admanager for serving ad inventory. If you are wondering what is Google Admanager, it is a hosted ad serving and management solution for publishers. Signin here with your Google Adsense account credentials

Today, we are pleased to announce general availability of the product — no invitation required!

If you have an AdSense account, you can sign in to Ad Manager today. If not, apply for an AdSense account now. A Google AdSense account is a technical requirement for creating an Ad Manager account.

Using Google Ad Manager you can server directly-sold ads as well as ads from Google adsense and other ad networks. You can setup ad slots for different placements in your site. You can use Adsense ads with other ad network ads, in this case Google will show the ads which will will give maximum revenue to the publishers.

Google ad manager is easy to use and is available in 32 languages. OpenX is an open source solution for this kind of ad serving technology, which i used before, but if you use Openx you have to maintain it by updating the code etc. With Google ad manager you can forget about maintaining your own ad server. Via Google Adsense Blog