Everyone knows that if a file is selected in a finder window that you can hit Command-O to open the file. But did you know you can also hit Command-Down Arrow to open the file with the default application for its type? Now you do!
[robg adds: This appears to be the opposite of Command-Up Arrow, which opens an item's enclosing folder. If you have a folder selected, Command-Down Arrow opens it ... or launches the selected application. As far as I can tell, this is an undocumented shortcut.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031123160824630