In alphabetical order:
Instapaper: An amazingly useful app, and by far the best way to read articles of any length on the iPhone. Tilt scrolling is a beautiful thing.
Kindle: I have a feeling that one of these days this could render my Kindle useless. If it weren’t for the discomfort that backlighting causes my eyes over long stretches of reading, it might already have done so.
WeatherBug Elite: By far the best weather app for the iPhone. Very fast and very good-looking, two things that can't be said for the other options that I’ve tried.
I have sometimes said that, because of lousy network coverage in my neighborhood, when my AT&T contract runs out I may well ditch the iPhone in favor of something with Android, or maybe even the Palm Pre. But who am I kidding? Apps like this, plus the ability to sync with my music and my other personal data, will keep me in the iPhone camp for the foreseeable future.