Examples of Online Databases:

The adoption of modern technology has resulted in the proliferation of machine readable databases. The National Library of Medicine (USA) designed its MEDLARS which became operational in 1964. Later appeared its online version as MEDLINE. Engineering Index of the Engineers° Joint Council (USA) and Biological Abstracts soon had their corresponding online versions, viz., COMPENDEX and BIOSIS. Yet another very large online database is that of Information Services for Physics and Engineering Communities, called INSPEC (UK) representing the three printed versions of Physics Abstracts, Electrical and Electronic Abstracts and Computers and Control Abstracts.

CA Search and AGRICOLA, the online counterparts respectively of Chemical Abstracts and Bibliography of Agriculture (NAL, USA) are worth mentioning. These are all also devoted to sciences. Of the social sciences, there exist a number of large online databases particularly in economics, psychology, political science and management. For example, PsychINFO is the online version of Psychological Abstracts. 

MARC database of the Library of Congress is a good source to search for resources in humanities. It is also worthwhile to know about two full text online databases: one is LEXIS in law and the other is NEXIS that offers several newspapers and business publications.