The Web of Science database is very useful for citation or cited reference searching. Citation searching is a specialised type of searching which allows searching for articles or chapters which cite a particular work. This indicates relationships between works and also to some extent the impact a work has made on others in the field. This only includes items indexed by Web of Science and has an English language bias. The Journal Citation Reports allow searching for impact factors which measure the impact of a journal and allows comparisons.
Ths Scopus database allows searching for citations and for journal rankings and impact factors (search under Analytics).
Google Scholar is useful for journal articles and Google Books for book citations. The sources indexed by Google are not clear, they may produce results such as a blog post, a lesson plan, a reading list etc. These may not be appropriate for pure bibliometrics.
Publish or Perish is a free piece of software that retrieves and analyses academic citations. It uses Google Scholar then analyzes these and presents the following statistics. You can see not just the number of times an article is cited but how many times per year it has been cited.
SCImago Journal & Country Rank uses algorithms which are similar to Google Page Ranking and is based on data in the Scopus database. It covers some arts and humanities subjects.