BASE - Bielefeld Academic Search Engine [Datenbank]  

BASE (Bielefeld Academic Search Engine) is one of the world's most voluminous search sengines especially for academic web resources, e.g. journal articles, preprints, digital collections, images / videos or research data. Other than search engines like Google or Bing BASE searches the deep web as well. The sources which are included are intellectually selected and reviewed. BASE is operated by Bielefeld University Library.

Web of Science [Datenbank]  

Web of Science is a curated collection of over 20,000 peer-reviewed, high-quality scholarly journals published worldwide (including Open Access journals) in over 250 science, social sciences, and humanities disciplines. Conference proceedings and book data are also available. Web of Science offers cited reference searching which means that you can search for records that cite a particular document.