This is a tip for Mac users. Use
M-x reveal-in-finder to show the current file in a new finder window. If you use this in dired, it will show the file at the cursor in a new finder window.
reveal-in-finder package may be installed by default with your emacs, but if not, you can install it through the package manager using
I find this useful for example to quickly attach files to emails in thunderbird. I just reveal the file in finder and then drag it onto my compose window in thunderbird.