Step One: New Canvas
Create a new canvas in Photoshop, roughly the width of your web page (or proposed web page) and roughly 30-35 pixels in height.
Step Two: Thin Rectangle
Grab your rectangle tool and draw a thin box across the bottom of your canvas. Rasterize the layer (Right-click the layer and choose "Rasterize Layer")
Step Three: Rectangle Tab
Once again, get your rectangle tool. Using this shape, draw a "tab" on the canvas. When you have it shaped and sized correctly, choose "Layer, Merge Down" to combine it into the previous layer.
Step Four: Foreground Color
Choose a foreground color that is different from the one you've been using, just to make your life a bit easier for the next couple of steps. (Click once on your foreground color, and choose from the color palette)
Step Five: Second Rectangle Tab
Draw another rectangle, this one about 1 pixel away from the sides of the first tab. It should be the same height as the first tab, but can extend below the long rectangle.
Step Six: Rasterize
Rasterize your new tab (Right-click the layer and choose "Rasterize Layer").
Step Seven: Duplicate & Position Tabs
Duplicate the new tab several times (Right-click the layer and choose "Duplicate Layer") and place each of these layers in the same manner.
Step Eight: Arrange Layers
Now, go back to your original tab layer and select it. Holding your left mouse button down, drag this layer to the top of your layer list.
Step Nine: Reference
Your secondary tab bottoms should now be "hidden" behind the original tab, making them perfectly straight.
Step Ten: Background
Now, go back to your background layer and choose a simple lined fill pattern to fill the background with. If you can't find one that you like, you could also use your line-shape tool to draw several light-gray lines across the background.
Step Elevent: Gels
Now we get to add the "gel". Select your original tab layer, and go to your "Styles". Click the arrow and choose "Mac OSX Gels". There are a ton of great effects here - choose a colored style of your choice and use it. (Play around with the styles - some are more transparent than others, and may show your secondary tabs).
Step Twelve: Secondary Gels
Going through each of your secondary tabs, use a gray/white layer style to finish the tabs. I used the "Mac Inactive Window" button because of its nice transparency to create the tabs below.
Step Thirteen: Add Text
All you have left is to add your text. Try using the Gel Styles for your text, as well - some of the "flat" gels create amazing text effects.
Play around with the different layer styles that you can find everywhere on the Internet. By simply using different layer styles on the tabs I'd already created, I came up with the following navigation tabs in under 5 minutes.
More Tutorials: 
