IRARGI, Centro de Patrimonio Documental de Euskadi (Center of Basque documental heritage), has been offering already for nearly one decade a good on-line database that grant access to historical documents kept in various archives in the Basque Country and elsewhere.

The webpage includes BADATOR, an excellent on-line database with nearly 500.000 historical documents and references.
Unfortunately, the pages and search engines are only available in Basque and Spanish.