How about User Fragging? All of originaltrilogy.com becomes an MMOFPS running a html5 port of some variant of the Cube engine (Cube, Assault Cube, Cube 2, Red Eclipse, Platinum Arts Sandbox, Tesseract etc...)...
On a serious note, I agree with pretty much every suggestion Frink posted.
The advanced text editor the site currently has is strange. It doesn't like mobile browsers, it REALLY doesn't like my Wii (I have to go to basic mode just to type anything) , it's difficult to put quotes in quotes...it's just...bleh. I like how the picture option allows you to resize the image to an exact resolution without physically resizing the image though. Even that can be improved by having an "automatically change value to fit aspect ratio if you change other value" option.