At least a couple of times a year I find myself searching on Google for this trick. In a nutshell, yes you can close a browser window that you didn't open (including the current one). Useful if you are writing an app in which you want to remove toolbars, etc. without popping up a seperate window and forcing the user to close the one you left behind. The gist of the trick is to set the window.opener = top and then execute a window.close. Very useful.
Another trick that comes in handy is to fake a modal dialog box by setting the onBlur="window.focus()" in the <body> of a popup. That will keep your popup on top (useful if you are asking a user to enter information and you don't want them to lose your popup by simply clicking back on the underlying page.)