I love black and white photography. Here are some ways to convert color digital images to black and white (or variations on the theme of low saturation tone) in Photoshop. There are special programs and plug-ins for doing this, but I find that Photoshop has all the flexibility I need.
The simplest, and least satisfying, method of converting a color photo to greyscale in Photoshop is to choose Image -> Mode -> Grayscale. It just strips the color information out of the file, converting each pixel to a shade of grey. You have not control over the process. This is not the right tool. Never use this.
A better way is to create a Channel Mixer adjustment layer in the layers palette. In the dialog box, you can choose the percentage of each channel that contributes to the final image (it's good to have the numbers add to approximately 100). Play around with different values until you get an image you like. This is a good way to do it, especially since the adjustment layer doesn't change the original image. You can change the channel mixer values, or go back to color, any time you want. You can also adjust the opacity of the channel mixer layer to desaturate the image to whatever degree you like.
Even more flexible (but more complex) is this method: first, create a Hue/Saturation adjustment layer (rename it to "desat"). Change its blending mode to Saturation. In the dialog box, move the Saturation slider to 0. Now the image is greyscale; big deal. But now create another Hue/Saturation adjustment layer underneath the first (rename it "tone"). Play with the Hue slider. As you do, you see a dynamically changing view of the image as colors used to create the grayscale image change. You can select for yourself the value that best brings out shadow detail or whatever you like. If you like a toned effect (like sepia, only with whatever tones you want), create a Color Balance adjustment layer on top of the two other layers. Try changing the color values differently for shadows, midtones, and highlights. For example, you can have a "greyscale" image with cool darks blending into warm highlights.