As far as I know there is not a complete hub the lists all the available datasets for NLP/text-mining. There are several (some outdate) that contain partial lists tho, such as
Thanks Ali for the pointer. We can indeed help you share it with the HLT community and give it some further visibility at ELRA/ELDA (http://www.elra.info and http://www.elda.org). You can have a look at our ELRA Catalogue (http://catalog.elra.info/) and the Universal Catalogue (http://universal.elra.info/) and get in touch with us for any further information (http://www.elda.org/article.php?id_article=68). We'll be happy to help! Kind regards, Victoria.