It's possible to drag and drop files to the Choose File button you'll see on some web sites -- like the one you'll see if you click the "Attach a File" button in Gmail. You do not have to click the Choose File button and then navigate the Open dialog; just drag and drop directly from the Finder to that button. This is quite handy when working with Gmail and attachments.
[robg adds: This was originally published in this post on the author's blog, where a screenshot provides some additional clarification, if you need it.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080410045258770