) textarea.

would be using textile-markup be an option?
you can try it here, for example: http://redcloth.org/
you just need the textile-output-filter for converting the textile-markup to html.
http://modx.com/extras/package/textilefilter

I think the Aloha editor may be suitable for font end use.
http://modx.com/extras/package/aloha

I don't think it matters, but I haven't implemented it yet myself.
Search the forums for "login aloha" or other similar specific search strings.
what your looking for is wysiwyg editors if i understand you... check for tinymce or cfkeditor (or whatever is the next-gen called) which transform your textareas with javascript into full text editors!
Sumoborac
I don't quite understand why the content itself is relevant.
Your users are editing profiles. Which editor you use, would be independent from the profile content.
How are you setting up your users?
[[!UpdateProfile? &postHooks=`photo,uploadPhoto` &photo_path=`****/profielfotos/pasfoto/` &photo_path_url=`******/profielfotos/pasfoto/` &photo_field=`picture` &photo_filename=`pasfoto-[[!+modx.user.id]]` &customValidators=`fotoValidatie` &validate=`picture:fotoValidatie` ]]