If you are using an iPhone, there is an exceptionally good app called HebrewBible, by Bill Clementson. It has the BDB lexicon in it, but unlike BDB it also contains a full concordance. You can search by the first two root letters, and when you have found the word that you are looking for it will give you a listing of every verse in Tanakh that features that word, together with the relevant part of each verse.
(I appreciate that this only partially "answers" your question, since it is not online and it is not free. I apologise for that, but my answer is too long for a comment and I thought that this app might be of assistance to you anyway, or to other people who read your question.)