Open iPhoto 08 and select the image you would like straighten.
Click the Edit button at the bottom of the window.
The image will now be opened in Edit mode. Click the Straighten button at the button of the window.
A slider will appear over the bottom of the image and a grid will overlay the image. Move the slider to the left or right until the image is straightened.
Click Done at the bottom right of the window to save your changes!