Updated: 20 April 2010
This tutorial will demonstrate how to create a sepia color tone using Adobe Photoshop 7.0. Although, the instructions are specific to PS 7 commands, similar commands should be available in newer/older versions of Photoshop.

Open your image in Adobe Photoshop and confirm that image edit mode is set to RGB Color.
menu: Image > Mode > RGB Color
Make a copy of the Background layer
menu: Layer > Duplicate Layer
(Optional) Auto-contrast the Background copy layer
menu: Image > Adjustments > Auto-Contrast (Shift+Ctrl+Alt+L)
Desaturate the Background copy layer
menu: Image > Adjustments > Desaturate (Shift+Ctrl+Alt+U)

Now we're ready to add the sepia.
Create a new layer (Layer 1)
menu: Layer > New > Layer... (Shift+Ctrl+N)
Set the foreground color in Photoshop to #A48464 (R:164 B:132 G:100)
Use the Paint Bucket Tool (Mode: Normal, Opacity: 100%) to fill in Layer 1

Add a Layer Mask to Layer 1
menu: Layer > Add Layer Mask > Reveal All
Apply Image to the Layer Mask on Layer 1
menu: Image > Apply Image...
... using the settings:

Select Layer 1 and set the Blending Mode to Color

Et voila! You should now have a beautiful, sepia toned image.
There are a number of further adjustments that can be done. With sepia toning, my favorite is to add a little saturation.
Create a new Adjustment Layer
menu: Layer > New Adjustment Layer > Hue/Saturation...
Adjust the settings to your taste, for example:
Elle est belle, n'est pas?
Wow, she is beautiful! Great tutorial.
Jandel