It is pretty easy to straighten images in Photoshop CS6. The new and improved Crop tool lets you straighten images as well.
- Select the Crop tool.
- Click Straighten from the control bar on top.
- Draw a line along an edge that should have been straight. For example, draw a line along the horizon, or perhaps the edge of a building. Photoshop will automatically rotate the image by the right amount, resize the canvas, and show you a live preview of what the final image will look like
- Press Enter/Return to commit the change.
Note: If you want the option to revisit your decision about the crop, uncheck Delete Cropped Pixels in the Control bar. Enabling this option will perform a non-destructive crop.