OpenX, a very popular ad-serving platform that is open sourced, has launched a version for very large publishers, (like Groupon-sized publishers):
With advancements in technology to buy ad inventory, OpenX felt it was time to develop tools for publishers. Until now, publishers either had a direct ad sales force or sold remnant inventory through third-party companies.
Publishers often find revenue management inefficient because they must use a variety of tools to coordinate different demand channels. OpenX Enterprise allows companies to combine the sales all in one platform.
We’ve considered it once (or twice) but it wasn’t what we needed at the time. Although we certainly not at the level of a Groupon it’s still very usable for a blog-network. You can be sure we’ve been keeping our eyes on it as they improved the platform!