Google Scholar:
Search engine for articles, theses, books, abstracts and court opinions from academic publishers, professional societies, online repositories, universities and other web sites. Searching is free, but links to full-text articles and books from the American University Library collections is restricted to American University users.
Worldcat (OCLC FirstSearch)
WorldCat can tell you how many libraries in the world own a particular book (and which libraries they are), which can be used to indirectly measure impact. Pay close attention to the results, as there are often multiple entries for the same book.