Hybrid Tango Blog has worthwhile commentary and YouTube videos. Their masthead says, "The secret of tango is in this moment of improvisation that happens between step and step. It is to make the impossible thing possible: to dance silence. This is essential to learn in tango dance, the real dance, that of the silence, of following the melody." Carlos Gavito
There is a website with a large selection of instructional videos, www.argentine-tango.com.