I saw this pop up on slashdot, so I thought it was worth a share 
Maqetta: Open Source HTML5 Editor From IBM - Slashdot
Release announcement: http://maqetta.org/index.php?option=com_content&view=category&layout=blog&id=2&Itemid=9
(note: maquetta.org seems to be up and down today due to high volumes of traffic)