I don't know if you could do this in previous versions of Mac OS X, but it certainly works in Leopard (10.5) with Safari 3. If you're on a site that has a File Upload or Choose File button -- one that opens up a file browser to select a file -- you can simply drag a file from the Finder to the side of the button, and it will automatically be used as if you had navigated to it with the file browser and selected it.

This can save quite a lot of time if you have your files on your Desktop or the Finder window open in front of you.

[robg adds: This works in Safari 3 in 10.4, too. Note that this is not the old 'drag to open nav window' trick; just drag to the general area of the button, basically.]