Personally I like Enlightenment... It is lightweight, portable, very configurable and has a very clean desktop. It has some very good functionality and effects that I like. You can check out more info at
http://enlightenment.org . You can also run KDE and GNOME on top of it... just pull up a terminal and type kde (for instance) and it will slide into place... close the terminal and it will close out the KDE interface. I like it personally.