Scopus (http://www.scopus.com/): Scopus is a relative newcomer to the scholarly search field, founded in 2004, but offers a great deal of flexibility for the bibliometric user. First, searches can be done on fields including the abstracts and keywords, but also on the references. This makes it particularly useful for the purpose of finding citations to digitised resources compared to the Web of Knowledge, which does not search the text of the citations. It also allows for relatively easy downloading of your searches, although there are some limits on very large results sets with over 2000 items.
Also, in Scopus has expanded humanities coverage in recent years, which makes this resource more valuable for finding citations to digital humanities materials.
Google Scholar (http://scholar.google.com): Free access.
Google Scholar is the easiest of the three main sources of bibliographic data to perform simple searches in, as the interface is nearly identical to the main Google search engine. Compared with Scopus and Web of Knowledge, however, you have far less control over your searches as Google Scholar does not include the ability to do fine grained boolean searching, and often returns far more false positives than the other services. However, Google Scholar also has the most coverage of informal scholarly communication (such as presentations and conference papers), so may be able to find results the other tools have not.
ISI Web of Knowledge (http://www.isiknowledge.com/): The Web of Knowledge (WoK) is the grandfather of search sites that use citation-based searching techniques. Founded by Eugene Garfield, one of the originators of many bibliometric techniques, WoK allows a variety of search options and the ability to follow citations from article to article. The databases included in the Web of Science portion of the WoK site cover the sciences, social sciences and humanities, and have recently expanded to include conference proceedings in addition to journal articles.
A major limitation of using WoK to find citations to digital resources, however, is that the fields you are able to search are somewhat limited: they do not include the full text of the article, and they do not include the text of the references.