When I am working on a project, I have occasion to get into one file and spend all my time on that file. In these cases, I like to have as few windows still open as possible. At those times when I have to have a Finder window open to find a file, I double click the file to open it and then Command-W to close the Finder, or click the little red button.
But I recently discovered a faster way to accomplish the same result. By holding down the Option key while double-clicking the application or file you want to open, the Finder window will automatically close as the application launches or the file opens. This might be useful to some.
[robg adds: We've covered using the Option key with the dock to hide apps while switching, but I don't think it's use in this situation has been covered here before. I searched the Finder's help files for any references to it, and while there are lots of Option key shortcuts, I didnt' see any specifically referring to closing a window while launching.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200309231118539