Tripcase is the most useful travel app I have come across. All you have to do is email your itinerary to firstname.lastname@example.org and it pulls out all the relevant info regarding your flights, car rental, hotels etc.
It keeps them all together in an easy to read interface, and you can update the trips by simply sending through new bookings via email. It keeps any eye out for cancelled flights, which is great if you are a grumpy traveller like me.
Keeping all my booking details in one place is one thing I no longer have to worry about, so long as I keep my iPhone charged and ready to roll.