Behdad and I prefer to have the "Original" size of a photo to view, or use (if it's under a CC lisence). So we wrote a Bookmarklet to make it easy to save the original size of an image on Flickr.
Here is the Flickr EasyDownloadr bookmarklet.
HOWTO: Right click on previous link. Bookmark it (preferably in Bookmarks Toolbar Folder and rename it if needed). Open the Flickr page of the image and use this bookmark to save the original size of the image.
UPDATE: And here are the Flickr EasyOpenr which just opens the original size.
 any later version. * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. * You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. * Copyright (C) 2006, Behname Esfahbod * Copyright (C) 2006, Behdad Esfahbod */ id=String(page_current_url.match(/\/[0-9]{3,}\//)).match(/[0-9]{3,}/); g=global_photos[id]; l=%22http://static.flickr.com/%22+g.server+%22/%22+g.id+%22_%22+g.secret+%22_o_d.jpg%22; document.location=l; ){kind=link}
 any later version. * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. * You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. * Copyright (C) 2006, Behname Esfahbod * Copyright (C) 2006, Behdad Esfahbod */ id=String(page_current_url.match(/\/[0-9]{3,}\//)).match(/[0-9]{3,}/); g=global_photos[id]; l=%22http://static.flickr.com/%22+g.server+%22/%22+g.id+%22_%22+g.secret+%22_o.jpg%22; document.location=l;){kind=link}