[prawn]

Sortfolio for apps. Smart (save for the site usability).

If you're bored and can't think of something to build over a weekend, hook up with a designer and emulate the Sortfolio concept for a particular niche. I've been looking at building an engine that can be rolled out across various industries, starting with things like weddings and landscaping/garden design. Any visually oriented industry would work IMO.