Remember TripIt, the travel-management service that organizes your itineraries? To paraphrase Apple's latest ad compaign, now there's an app for that. TripIt for iPhone [iTunes link] helps you manage and share your travel plans.
Specifically, the app provides access to your TripIt itineraries, even when you're offline. The app itself is free, as is the TripIt service.
In case you're unfamiliar with the latter, here's how it works: After signing up for a TripIt account, you simply forward all travel-related confirmation e-mails (i.e. those you get from airlines, hotels, car-rental outfits, etc.) to email@example.com.
The service culls all the pertinent details from those e-mails and builds detailed itineraries, which you can then share with others (great if you need to keep, say, your boss or spouse in the loop on your whereabouts).
TripIt for iPhone lacks a couple of bells and whistles. For example, it doesn't provide access to itineraries for "shared trips," and it doesn't offer "closeness alerts" (meaning it can't tell you when other TripIt users are nearby, which would seem like a natural capability for an iPhone app). Even so, I'd say this is a must-have app for TripIt-using iPhone owners.
For everyone else, there's always TripIt Mobile, which provides a streamlined Web interface for most mobile Web browsers.