Promopicker is a broadly general podcast directory that offers a few extras in information they collect and post.
They use a two level hierarchy of topic categories.
One can add their podcast using the Suggest Link to this category on each category page.
It is in the show listings where they offer extra information. First the show listing shows the expected show name, link, description and categorization. Additionally they offer a link to a MySpace page, display "album art", and list sample episodes or promos.
I suppose that listing the promos would help someone browsing the site decide whether to subscribe. However the promopicker site also has an embedded player, so someone can listen to episodes without subscribing.