An obvious play is Yahoo, which works very well for many advertisers. That's why the company typically sells out its Christmas inventory months before the holiday season arrives. If you're a retailer offering a weekend sale and you want to reach a large number of people in a short time, pick up a whole page in Yahoo, or buy every impression on the site for a fixed period of time. In the U.K., 8 million people will see your message the first day; in the U.S., upwards of 20 million.

One feature that sets Yahoo apart from other advertising buys is its advanced approach to segmenting visitors. Like most other sites, it factors in the obvious considerations like the previous content the user has been browsing and recent searches the user has conducted. But Yahoo also applies proprietary algorithms that seek to measure the intensity of a user's interest in a topic, and it offers indications of how close the user might be to making a purchase decision.

MSN is another household name among websites, and it can be a great buy when you want to develop a wide reach in a very short period of time. For example, suppose you are trying to launch a new product and your strategy is to hit as many people as possible, something you might try to accomplish is a TV-style broadcast campaign. MSN can be a great online buy for this classic strategy, cost-effectively providing exposure of your advertising messages to massive numbers of unique visitors in a single day.