Pt_extlist is one of the two requirements when you install the YAG plugin in a TYPO3 project. I guess, most of you install this extension without knowing, what the extension is good for. Extlist is a generic list generator, which enables you to build a filterable, pageable and sortable frontend list by only using typoscript. (You find a short introduction here.) All listings used in YAG are powered by pt_extlist.
Till now, we only used a subset of the provided extlist features. But the uncached imageList plugin mode introduced with YAG version 2.3 enables us to utilize a lot more extlist features.
As a showcase, i built a tiny theme called "Library". This theme uses a set of option list filters, a tag cloud and a generic search filter to select from the complete set of images of this website.