Your particular machines aren't up to it, but my single favorite apple application is Aperture. Yes, it's not as stable as I'd like it to be, yes it saves all your changes in it's own format so you're stuck with it for your editing, but it's just so damn slick at importing, adjusting the various levels and white balance, cropping, and exporting out in whatever format you might need.
As to killer apps for your machines, I can't really think of any. I'd still love to own a iMac fishbowl, but I can't bring myself to sacrifice the old CRT imac that I was given because the damn thing still works. The real killer app is that when you want to do something tricky, it's all nicely unix based, so things work like you expect them to. Of course, seeing as you run a linux desktop as an alternative, that's not much of a change.
Matthew