My own contribution to the Mac OS X Resolution Independence project is called Rectiffy. It came from an idea proposed by Rory Prior to write a small app that lets you add and remove images from a TIFF file. TIFF files can hold multiple representations of the same image, like icon files, but the representations can be any size and have any DPI setting.
Rectiffy has functioned as a viewer for a while but today I checked in my first three-hour rush job shot at editing the representation list (adding and removing) as well as outputting the final TIFF. If you’re interested you can check out the Subversion repository (it’s experimental but it seems to work) as long as you understand that it’s not really pretty yet, but it does do things like check the images you add for having the same aspect ratio. And if you want to check some of these TIFFs out, Coda’s TIFF resources are mostly (if not all) multi-representation TIFFs.