I really like Gimp since it allows you to set the Monitor Resolution that you are working on. OSX, etc assumes a screen resolution of 72 ppi when viewing images at Actual/Original size. More often than not this isn't the case. For instance on my 12" powerbook the screen is 9.68" x 7.26". At a resolution of 1024x768 pixels this gives: 1024 / 9.68 = 105.8 ppi. No where near the assumed 72 ppi.
It can be really handy to have an image on screen at the actual size it will print. For me it was printing a picture the correct size to fit into a picture frame for a Doll's House (it's not my Doll's House, it's Mimi's; but I do play with it - I have to!). It was handy to hold the frame over my display and see that the image was going to come out the right size
To me this seems pretty fundamental stuff and I'm surprised Apple don't include this scaling automatically. Since they control software and hardware it should be relatively easy to do, or at least include a system wide user defined option
Anyway, next is figuring out a way to do the same in Preview.app