Hi Jelle,
I still use htmlarea in a project since I have spend a lot of time apapting it (being not a fan of javascript, but in the end I found it was possible to modify it, maybe easier than some others), but when looking around for alternatives this one seemed quite promising:
http://www.kevinroth.com/
It supports browsers beyond IE, and the html can be transformed to xhtml (this has always been bugging me with htmlarea, and since it makes use of the DOM it is ways more elegant than my beautification approaches with php).