There is a great article on MTBR about their best iPhone apps, here is what they found:

5 - “The Bike Computer” ( http://www.everytrail.com/iphone.php) FREE - Provides a large display of speed and distance based on your iPhone’s GPS. Also contains a bunch of other GPS based features, but the clean and simple bike computer is the best part.

4 - iTunes - keeps you rockin’ in the woods! It’s already on your iPhone, just load it up with your favorite songs and go.

3 - “Clinometer” ( http://www.plaincode.com/products/clinometer/ ) $.99 - For measuring head angles, seat tube angles, etc… (thanks to Scot Nicol for the tip on this one)

2 - “MotionX GPS” ( http://gps.motionx.com/video/ ) $2.99 (free lite version also available) - For biking and almost any outdoor sport enthusiast. This is a well developed application with tons of features but is still easy to use. Very slick user interface.

1 - “Bicycle Gear Calculator - Bike Gears” ( http://www.jpmartineau.com/iphone/bicycle-gear-calculator/ ) $4.99 - A classic bike app, use it to calculate gear ratios, gear inches, and more.

Have a look at the original article on MTBR.

Runkeeper ... Works like a charm!

I use the Accuweather for the weather, I also use the Notes application to have checklists for riding.