If you surf the Web—and if you’re reading this, you obviously do—you may have noticed a new breed of online ad recently, appearing on such marquee sites as NYTimes.com. Just in time for Halloween, it springs unexpectedly from a seemingly normal banner ad like a ghoul jumping out of the bushes and shoves whatever you’re looking at to the bottom of the screen (don’t worry, it crawls back into its shell after 7 seconds). It’s called a push-down ad, and it was introduced this summer by the Online Publishers Association (OPA), a trade group that includes most of America’s biggest media companies and is devoted to promoting Internet advertising.

The online-ad business is no longer in its infancy—more like its adolescence. And like many adolescents, it is going through an experimental phase. “We were hearing from the agency community that they wanted new ways to connect with the consumer,” says OPA president Pam Horan. “If you’re trying to deliver a unique brand experience, this provides a new way to do that.” The push-down has been very successful so far, according to Horan: Several of the companies that tried the new ad unit when it was first rolled out—a group that included such names as Porsche, Air France, and Levi’s—have already signed on for followup campaigns.

The difficulty for advertisers is finding that sweet spot between being annoying and being ignored. On one hand, pop-up ads are generally considered such a nuisance that virtually every browser comes with software to block them; on the other hand, some marketers fear that Web surfers have trained themselves to ignore traditional, unobtrusive banner ads (there’s even a name for the supposed syndrome: “banner blindness”). Push-downs definitely grab your attention, but is there a risk that they’ll become as reviled as the pop-up? According to Horan, the OPA has not received complaints about push-downs, and as she notes, “Consumers are not shy. If they have an issue, they let us know immediately.”

The push-down is just the latest in a long line of advertising innovations aimed at wringing more money from the Web, where ads are still exponentially cheaper than they are in print or on TV (Horan confirms that push-down ads are “commanding a premium”). Here’s a quick primer on some other favored techniques, old and new:

1. The Big Brother Ad

One of Google's main sources of revenue is targeted ads based on keywords that you enter into its search engine or its email program. But while nobody minds when they Google “aluminum siding” and a list of home-improvement-store ads shows up alongside the results, it can feel like a rather disturbing invasion of privacy when—as happened to one BoingBoing commenter—you are having a Gmail conversation about the fact that your child was born without a left hand and you get an ad reading “Table Saw Accident?”

2. The Stealth Ad

The popular aggregator The Daily Beast, which for months after its high-profile launch carried no ads whatsoever, is trying out lots of novel ways to integrate ads and editorial content seamlessly. One of several nontraditional ad types they’ve recently showcased is what they call a “homepage expandable unit”—an ad subtly disguised as one of the eight rotating “box stories” on the homepage.