Photobooth is a Mac app that lets you take pictures using your web cam. It includes effects to dress up the pictures.

I didn't realize this until recently, but there's a similar app for Linux distributions running Gnome, like Ubuntu, that can be used for the same thing. It's called Cheese and you can read about it here: http://projects.gnome.org/cheese/.

To install it go to "Applications" > "Add/Remove..." and then find "Cheese" in the list of applications. The installation is pretty obvious from there.