Itinerary Organizers: Apps like TripIt (free, Internet) save all your trip details in one convenient place. Forward reservation emails to your TripIt account, and it automatically adds them to your itinerary.

Travel Booking: Skyscanner (free, Internet) helps you survey a wide variety of European budget airlines to find the cheapest connection between any two points.Orbitz, Priceline, Booking.com, Expedia, and Travelocity (free, Internet) allow you to search for flights, hotels, rental cars, and more. 

General Europe: Rick Steves Audio Europe (free, self-contained) has hundreds of radio interviews and dozens of audio walking tours of Europe's top sights, organized by destination for easy browsing. Download the playlists that interest you and fit your itinerary before your trip, then listen to them offline as you travel.

Public Transportation: Various subway map apps for the London Tube, Paris Métro, and others (pay, self-contained) have detailed, digital plans of public transit networks that save you from having to unfold an unwieldy map on a busy platform. The free, self-containedMetrO is a handy route-planner for public transit in dozens of European cities, but does not have maps.