GUI Mockups with Pencil

I came across a nice little free tool for building quick GUI Mockups: Pencil. It certainly has anything you might need for getting a flat prototype done—which is what I am using it for—but you can also go crazy and link multiple pages, export to multiple HTML pages, etc.
The only feature I am missing so far is selecting all elements in an area with the mouse, but overall things work out better for me with Pencil than with Viso or alike.