How to extend "filtered html" by ability of using class attribute? I can't let user have full html, but classes seem to be useful.
I use the Wysiwyg filter module in combination with the wysiwyg module for this. The module allows you to define exactly what properties to allow, what elements, classes etc. It takes a little time to set up, but the added control is well worth it imo.
I found way of implementing of filtering system: http://definitivedrupal.org/node/90
but I still hope there's easier way like just a checkbox [x] don't filter class attribute.