The iTunes App Store of Apple has an extensive array of travel-related applications that you can install in your iPod Touch, iPhone and iPad. Many of the apps are free (or at least available in ad-supported and "lite" versions) and others can be bought for as low as $0.99. You must have an iTunes account before you can buy the apps.
Here are some free apps I would recommend for tech-savvy travellers:
TripIt - this app consolidates your travel itinerary by parsing your booking information into a organized list. After downloading and installing the app, you will be asked to open an account in their website, after which you email them your booking info that will be used populate the app.
Frommer's Travel Tools - this is a collection of functionalities, including a converter, time translator, tip calculator, a packing list function, flashlight and more.
HostelWorld - this free app from HostelWorld.com is used to search and book rooms and accomodations. It can save important data such as directions and booking info for each hostel.
Currency - a free app for an at-a-glance currency conversion. You can customize the currencies to include in the list. The app updates the conversion rates when there is an internet connection, either via WiFi or 3G, but it will use the last info when offline.