Google Scholar is a freely-accessible web search engine by Google that indexes the full-text of scholarly literature across a very wide array of publishing formats and disciplines.
Explore the ISI Web of Science database with multidisciplinary coverage of over 10,000 high-impact journals in the sciences, social sciences, and arts and humanities, as well as international proceedings for over 120,000 conferences.
PubMed is a service of the U.S. National Library of Medicine that includes over 18 million citations from MEDLINE and other life science journals for biomedical articles back to the 1950s. PubMed includes links to full text articles and other related resources.
UniCat is a union catalogue of Belgian libraries. It's the successor of the CCB (Collectieve Catalogus Begië). It currently holds some 14 million records from the main Belgian university libraries and the Royal Library.
Antilope is the Belgian union catalogue of periodicals. You can search it to find a library holding the specific volumes/issues you are looking for.
WorldCat lets you search the collections of libraries in your community and thousands more around the world. OCLC network grows every day.