GMail has added the drag and drop way to attach files to email messages. If you have your folder containing the files to be attached open, select the files and drag them to the Compose page’s Subject area. Below is a screenshot of the process.
This definitely saves time as oppose to the old fashion way of browsing through your folders to locate the files to be attached. But according to their blog, only Google Chrome and Firefox 3.6 supports this feature as of now. Other browsers will be supported in the future.