Some sites like WordPress and Google+ are adding the ability to drag and drop files to upload them. But if you use Safari, you can drag and drop onto most file upload buttons.
If you have to upload a file to a site and you see the standard rectangular “Choose File” button, you can drag and drop the file from the Finder onto the button. Be specific, though—when your mouse is over the button, it should turn blue to indicate that it will accept the impending operation. If you drop the file anywhere on the window, or just outside the button, it won’t turn blue and Safari will attempt to open the file in the current tab.
This is really handy if you already have the file on your desktop or buried somewhere in a folder, but opened in a Finder window.