This is an idea i've been meaning to code for a long time but as it's not really my area of expertise I thought i would put it out there in case anyone else was interested.
The idea is for a program that would let you drop in a bunch of photos and would let you move around their thumbnails to arrange them in some kind of grid, and then save the full sized montage.
An important aspect would be snapping behavior. If you have ever used the windows multiple-monitors settings dialog, that is what i had in mind:
This would let users easily arrange photos in a horizontal strip, vertical strip, grid, etc.
I can see some additional nice flourish options such as a function to autofit a grid, options for some kind of automatic bordering feature, resizing options, and ability to drag and adjust relative sizes of images.
This is a program that is 90% user interface.