Recently I've created a grid based layout for Panels that features intelligent (well, kind of) adaptability. It renders the output based not only on how panes are layed out but also on how empty slots are distributed. It means that panes automatically expand to the right to fully occupy empty cells in the grid. Thanks to this behaviour very flexible auto-constructed layouts are possible.
The project is hosted on Github http://github.com/bdziewierz/Three-Four. Check it out if you're interested. Comments welcomed!