If you have upgraded to Photoshop CS3 or CS4 or if use Lightroom, then you have encountered Vibrance. Consider it a smarter less heavy handed form of Saturation. Saturation will saturate all colors equally, and it is easy to go over the edge into blobs of frightening color and loss of detail in a color that was pretty saturated to start with, just to get the saturation up on another color that started out less saturated. Vibrance on the other hand will saturate less-saturated colors more than ones that are already more saturated. It will also protect somewhat against oversaturating skin tones. Consider this example. The first image is before adjustments. My goal is to increase the saturation of the colors in the hat.
When I increase saturation, as shown in the second image, it not only increases the saturation of colors in the hat, but the skin gets yellow as well. In addition, the reds in the hat have started to go over the edge where you lose the detail of the yarn, because the reds were more saturated than the blues to start with.
When I instead increase vibrance, notice that the skin color is pretty well preserved, and that I am able to get more saturated blues without losing the detail in the reds.
How well you see these changes will depend on your monitor. Give it a try with your own images, comparing what you get with saturation and with vibrance. I find I rarely use Saturation any more.
Photoshop CS3 has Vibrance in Camera Raw. CS4 also introduces a Vibrance adjustment layer.