Adobe Photoshop contains tools like the background eraser and an extract filter to help you quickly erase complicated background images from digital photographs.
Want to do something similar but without Photoshop ?
Just upload your picture to FotoFlexer, an online photo editor, and open the Geek tab. There you’ll find an intelligent tool called "Smart Scissors" that works much like the Magnetic Lasso selection tool of Photoshop.
Draw a rough boundary around the image that you want to separate from the background and you will notice that "Smart Scissors" magically snaps the curves to the nearest edge (see screenshot on the right).
Once satisfied with the selection, click "Create Cutout" and you’re done.
You can use this free FotoFlexer service to extract picture of a particular person from some group photograph or you want to merge two photos – cut a portion from the first image using Smart Scissors and place in over the second picture.
The cutouts can be saved as JPGs or a PNG so I guess it maintains the transparency as well. Have fun.