Dropshadow Effects
Websites that will add a dropshadow to an image
cutmypic
Pretty capable. Shadow can only go to the bottom right, but it's adjustable at least with respect to color and spread.
anymaking
Allows you to set background and shadow color and adjust size, opacity and distance of the shadow. Pretty good. Even better would be if it let control shadow direction and wasn't written in flash.
lunapic
You get exactly this dropshadow pictured here and you can't change it. It is really simple to use though so I guess there's that.