Google Scholar is a free citation resource. It counts citations from academic articles, theses, books, and other documents.
Search citations for individual articles from the homepage. Researchers with a Gmail account can set up a Google Scholar Profile in "My Citations." It keeps track of citations and retrieves author metrics (h-index, i10-index). "Metrics" leads to a summary of recent citations to many publications in broad and specific research areas, which can help authors decide where to publish.
Below is a screenshot of Google Scholar homepage: