Photoshop CS4 has a great new feature called the Pixel Grid. The pixel grid appears when you zoom past 500% and can help with editing at the pixel level.
You can control whether this grid is displayed or not using the View > Show > Pixel Grid menu option.
If you don’t see the pixel grid menu option, then you most likely haven’t got OpenGL enabled in your Photoshop preferences. To fix this, select the Edit > Preferences > Performance… menu option. Make sure that Enable OpenGL Drawing is checked.
I personally find the Pixel Grid really useful. Do you? Leave a comment, I’d love to hear.