Apart from using the Web of Science citation index, the following citation databases are available, free of charge, to do a targeted search for citations:
Academically-oriented search engine and citation databank. As well as citation verification, a search gives you bibliographic information, links to full texts, text excerpts, abstracts and related literature. The citation verification indicates where and how often the requested literature has been cited. The advanced search allows you to search by author.
Harzing's Publish or Perish
This is a software program for retrieving and analyzing academic citations, using Google Scholar. You can download the Windows version of the program here.
The world's largest search engine for scientific information. Apart from scientific documents and web pages, references are also collected so that citations can be verified. The advanced search allows you to more detailed request.