Chrome: Google Docs/Drive has offered drag-and-drop for file uploading for quite some time, but you could only upload individual files, not folders. Finally, Google has rolled out this timesaving feature for folders.
It works as you would expect. Simply drag-and-drop folders into the Google Drive web interface, just as you would to upload a file. Note that you'll need to use the latest version of Chrome for this to work.
Drag-and-drop to upload folders in Google Drive [Google Apps]