Bertsolaritza, improvised poetry
Oral Tradition, a scholarly journal by the University of Missouri, has published an issue fully devoted to Bertsolaritza, the Basque improvised poetry phenomenon. Several Basque contributors fill most of the number, but as an introduction for foreigners, perhaps you should also read the outsider view by John Miles Foley, the director of the Center for Studies in Oral Tradition and the Center for eResearch at the University of Missouri. That article, like all the others, is available as a PDF in the journal's website.
In this video from Bertsoplaza.tv, a site powered by Plone, one of the many versions Bertso-making can have: question and reply, in a bertso-dinner. The tall guy reads his pre-writen lines (around the topic of Streap-tease), and the bertsolari, Jexux Mari Irazu, improvises his replies immediatly.