Here's an image-tut I created last year when vectoring was the craze at the forums I visited. Some people have shown interest in learning the pen tool to crop (aka 'render') out objects and such out of images.
Reading the little tut below should give you the basic knowledge of how the pen tool works, and how you can take advantage of it to crop things out cleanly.
If you have any questions, feel free to ask :)
EDIT: Once you made the path around the object, be sure to Right Click -> Make Selection, press Ctrl + I (inverses selection), and Ctrl + X (cuts the selection out of image).