Ever seen those calendars (I believe they're by an artist/photographer named Kim Anderson) that have a black and white photo with certain parts of the photo in color? Usually, the colored object is something like a rose or a person or something. It's a technique that brings a lot of focus to a specific person or object (the movie Schindler's List also used it).
Creating this effect is extremely simple, and there are a couple ways to do it, both very similar. The first method is easier to understand, but the second method gives you better flexibility.
Method 1
Take any color photo, copy the layer so you have two duplicate colored layers, one on top of the other. Select the top layer in your Layers palette, and desaturate it (Image >> Adjustments >> Desaturate).
Now you should have a B&W layer over a colored layer. Switch to your Eraser tool, and make sure the mode is set to Brush. Select a good brush size, and begin erasing the object you want to be colored. You'll erase the B&W "version" of that object, and the colored version on the bottom will show through.
Method 2
Open up the color photo and create a new, blank layer on top of it. Select this layer and fill it with black. Now, go to your Layers palette, and change the Layer Mode to Hue.
Now, do the same thing as in Method 1 - erase the object's shape from the top black layer, leaving the colored one underneath to show through.
This method has a couple advantages. First, if you want to save the PSD file, it'll be a lot smaller than Method 1.
Second, if you screw up while erasing and you erase outside of the object, don't worry about it - keep erasing. Later, select your regular Brush tool, change your forecolor to black, and paint black over the extra colored part - it'll go back to being black-and-white.
EXAMPLE 1
EXAMPLE 2

(I take pictures of everything - people look at me funny when I take these types of pictures - this one was in San Francisco across the water from Alcatraz. Notice the bottom row of bricks - I erased/colored them all completely first, and then re-added a black-to-transparent gradient to make it "fade into color")
Bonus Tip
You can change the opacity of the black layer to allow subtle hints of color into the picture. This can create a nice effect, too. Try out either method on a picture, then after you're finished, set the opacity of the top/black layer to about 80%. This is a great effect for getting that "traditional" feel.
Another Bonus Tip
Want an even more traditional feel or a more "emotional" feel for the image? Duplicate the color layer, select the new, duplicated layer, and apply a Gaussian Blur of about 1-10 pixels, depending on the size of your image. If you're working with a 300x200 photo, use 1-2 pixels, 900x600 pixels is about 5 pixels, and so on. Now reduce the opacity of the duplicated layer to about 30%. This creates a very soft blurry glow on the image, which works really well with B&W photos.
One Last Bonus Tip
Take the blurry layer from the previous bonus tip, and change its Layer Mode to "Overlay" for more dramatic/contrasting lighting. The Example 2 photo was a little "blah" before I did all this.
- Jonathan