For those of us who think that ad exchanges represent an exciting, fundamental shift in the way much of online advertising is bought and sold, today was the day we’d been waiting for:  Google announced the launch of the re-vamped of the DoubleClick Ad Exchange.

At the moment ad exchanges (auction-based marketplaces that match sellers and buyers of display inventory) represent only about 10 to 15% of the display market, according to the NY Times. But expect that percentage to spike as a result of the new DoubleClick platform, for two reasons:

1. An influx of new inventory.
Many of the top websites already use DoubleClick’s DART for Publishers product to serve their ads, and the new exchange makes it possible for them to easily put unsold inventory up for auction in the exchange. The new exchange will also include Google’s AdSense publisher sites, which reach 76% of the internet audience.  The combination of the two means the exchange now provides access to the full spectrum of display inventory, from premium to long-tail.

2. A familiar and easy to use interface.
One of the factors that’s limited the growth of exchanges is that they just have not been easy enough for buyers to access and manage. As part of the re-launch, DoubleClick Ad Exchange inventory will now be available through Google’s AdWords product, which is familiar to anyone who’s ever bid on a search keyword. In other words, Google just removed all barriers to entry to buying display ads on the exchange.

So, a very important for exchanges: the market just got bigger, and more accessible.

Note: For those who have been closely monitoring the exchange space, it’s worth pointing out that two of the most interesting features of the DoubleClick exchange – real-time, impression-level bidding, and the ability to append data – can’t be enabled through the AdWords interface. For those features you need direct access into the exchange, which for most agencies will come through a partnership with one of the “demand-side optimizers” like DataXu, MediaMath, or Invite Media.