Lots of them out there. PhotoShop. Not free. Gimp, free
and many others. If you have Windows there's a free program in that.
Just make a copy of your image in what ever program. Keep it on a separate layer if able. Darken as much as you can. Soften the edges a bit and then control the amount of transparency. Reposition it with perspective yadada. Or you can find a easy to use plug in for you image program. There are many ways to do this. Here is a link to start. http://www.mehdiplugins.com/index.htm
Oh and don't forget to check out the many useful threads here that provide those free programs too.