Since 2009, Google has had AdWords Sitelinks and since then they have toyed with different formats.

Brett Tabke posted a thread at WebmasterWorld linking to a Google FAQ page on the various ads that qualify for sitelinks and the type of sitelinks available. It reads:

Ad Sitelinks can appear in four different formats: three-line, two-line, one-line, and embedded:

Three-line and two-line formats: Sitelinks are designed to trigger in situations where an ad provides the ideal answer for a search query. These ads are most likely to trigger on unique brand terms.
One-line format: Sitelinks will trigger with more generic terms, but may also include brand terms.
Embedded format: Sitelinks will trigger whenever your ad qualifies to appear above the search results and the text in your ad exactly matches one or more of your Sitelinks.

The following best practices will help increase your chances of showing Sitelinks:

Your ad should have one of the top positions above the search results.
Your ad should have a very high Quality Score.
Your Sitelinks URLs must direct users to pages that are part of your main website.

The WebmasterWorld thread has advertisers discussing if they like this or not. Some do and some don’t. Do you?

Forum discussion at WebmasterWorld.