Google Adsense is testing collapsible ad units and I had an opportunity to check them out. Typically there were 4 links in the large rectangle 336×280 px ad unit. But what was unique was that only the top ad was shown in full. Rest of the 3 links were collapsed. Hovering over those links, collapsed the previous ads and made the new ad active with full display of content.
See how the ad unit looks like
And see what happens when you hover over the other links and not click it. Note that the ad unit changes color to gray background when you hover over it. Also note that the url is shown when you hover over the arrow.
I did not click the ads but I assume that clicking on them will take you to the advertisers website. It again shows the importance of the ad link title which will prompt users to hover and click the link.
I particularly like the large prominent links which truly stand out on any page and will encourage a high CTR. Have you seen these ads? What are your thoughts about these ads?