Traditional scholarly output is measured by citations, which can be found in a number of databases. Most notable sources for citations are ISI's Web of Science, Scopus, and Google Scholar. You will also see citation counts in databases and on many journal publishing platforms.
In addition to the sources listed above, there are a number of subject area databases and journal publishers that also provide citation counts. Some of these other citation sources include:
NOTE: all of these citation counts will be incomplete, because they can only count citations they can see. There is no single comprehensive source for citation counts.
Google Scholar is a free search engine that indexes a variety of scholarly information, including articles, theses, books, preprints, abstracts, and technical reports from many different subject areas. Google Scholar searching generally indexes more sources than either Web of Science or Scopus.
Waiting to discover your article citations or the impact of your journal can take a significant amount of time. Alternative metrics, or altmetrics, allow researchers to collect data immediately on how their research products have been used. Altmetrics collate a variety of metrics, including the number of views/downloads, social media activity, mainstream news coverage, and usage among online reference managers (such as Mendeley).
Each company providing metrics come with their own representations of the data, so you can select the resource that best matches your own needs.
Altmetric seeks to track and analyze the online activity around scholarly literature. Though some of their services are fee-based, their Altmetric Bookmarklet is a free service that provides metrics on recent literature.
Plum Analytics can be found in EBSCO databases.
PLoS Impact Explorer provides altmetric information for articles in the Public Library of Science.