It is called Selective (or partial) desaturation.
1. Duplicate your background layer.
2. Desaturate that layer (i.e. make it b&w).
3. Apply a layer mask to the layer. Use the black brush to paint on the layer mask (make sure that is where you are, not on the image itself). That will bring the color back to those areas. If you make a mistake, change the brush to white and go over it.
4. When you're happy, flatten and save.
If you don't have any image editors, you can do it for free on
http://www.picnik.com in their effects section.
Here is how to do it on picnik.com:
Upload a photo
Click on the "Create" tab
Click on "Black and white"
In the "Effect Painting" box, choose "Original"
Choose a brush size, then simply paint the color back in where you want it.
If you make a mistake, change it to "Effect" and go over the mistake.
Save the picture.
