Advent: A free version of Colossal Cave Adventure, on the iPhone day one.
Phonesaber: Yes. This is exactly what you think it is. And it's free.
Light: Don't spend money on one of those other flashlight programs -- get Erica's original for completely free. (In fact, you might as well go ahead and get all of her apps, they're all great.)
MixMeister Scratch: Free vinyl scratching over your iPod touch or iPhone's background music. Silly, but fun for aspiring (or, more specifically) wannabe DJs.
Zenbe Lists: A pretty good-looking free alternative to all the other To-Do applications out there.
???: I have no idea what this is (looks like some kind of Memory-type game), but it's free and it's hilariously beautiful. Update: Actually, this looks like the non-English version of Jirbo Match. But still, it looks like amazing and colorful fun.
NetNewsWire: This and AIM aren't really hidden, but they're must-haves, of course.
Have more that you've seen? Link us in the comments. I may not have the ability to install apps on the phone yet, but when I get it, I'll be ready.