Here is a Free Retina Icon Set, which will help each iPhone or iPad application developer make high resolution GUI for Retina displays. The icons from this set can be used in navigation, tab bars and toolbars, as they specifically meet the icon guidelines for iOS application design.

A wide range of image sizes is included into this collection. It allows to use them with iPhone 4 and iPhone 5 retina displays. All the images have a transparent background and are available in two colors: black and white.

Technically, there are unique app icons created in matching style in a PNG format. The sizes for navigation bars of the following devices are available: 20x20 and 40x40 pixels (high resolution) for iPhone and iPod touch, 20x20 pixels for iPad. The tab bar sizes include a 30x30 pixels icons for iPad and 30x30 and 60x60 pixels icons for iPhone and iPod touch.

All the icons from this set are available for immediate online preview. A trial download is also provided for your convenience.

After you've decided on the appearance of your icon, follow these guidelines as you create it:
Use pure white with appropriate alpha transparency.
Do not include a drop shadow.
Use anti-aliasing.
If you decide to add a bevel, be sure that it is 90 degrees (to help you do this, imagine a light source positioned at the top of the icon).

For toolbar and navigation bar icons, create an icon in the following sizes:
For iPhone and iPod touch:
About 20 x 20 pixels
About 40 x 40 pixels (high resolution)
For iPad:
About 20 x 20 pixels

For tab bar icons, create an icon in the following sizes:
For iPhone and iPod touch:
About 30 x 30 pixels
About 60 x 60 pixels (high resolution)
For iPad:
About 30 x 30 pixels

These icons are here: http://www.aha-soft.com/free-icons/