This might be rather dull hint but, while playing with the keyboard in Panther I discovered that if you Option-click any file, folder, or icon on the left sidebar in the Finder window, the item will open in a new window while closing the previous window.
[robg adds: If you just want a new window, keeping the old window open, hold Command and click.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003102721015327