Ad Supported Models Versus Subscriptions

One theme I have been seeing a lot of lately are free versions of web sites or services that currently cost money. For example, WebEx is a paid service, but there is a very smart and motivated entrepreneur developing a free (ad supported) equivalent. Match.com is a paid service, but there are two big ad-supported versions with hundreds of thousands of users. In the reviews space, tripadvisor.com is an ad-supported and referral fee based service (e.g. they get paid a referral fee if you book travel through them, but it costs nothing to the user). But where is the free version of Zagat.com? Search for "restaurant reviews" on Google and you'll find CuisineNet, restaurants.com, and a few others. Some of them provide helpful information, but none of them seem to provide the ease of use and authoritative-ness of Zagats. Could you use Drupal as the basis for an ad-supported equivalent?

Lots of these businesses are very interesting, and this is a clear category: creating ad-supported equivalents of services that currently cost money. Are there other paid categories, either online or off, that are really big enough to require venture funding?