If you've seen photos which have been edited to keep only a portion in color, then you've seen something called selective saturation. I recently discovered these images, and wanted to play around with them, but I couldn't round up the chips to go out and buy Photoshop CS4 for such a thing. Playing around with Preview, I stumbled on this method.
Open your original picture with Preview and select Tools » Adjust Color. Drag the Saturation slider all the way to the left to make the picture grayscale. Use File » Save As to save the modified image.
Now open the original (color) picture again, and select the Lasso Selection tool from the Select item in the toolbar. Use the lasso to surround the area you want in color, then copy the selection. Open (or switch to) the grayscale version of the image and paste. Drag the section to where you want it to line up, and use arrow keys to fine tune. When done, select File » Save As to create your new selective saturation photo.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090905230434451