Using a cited reference search allows you to find articles that have quoted a given article in their bibliography. A citation search takes you forward in time, by identifying more recent articles that cite an article of interest to you.
The Web of Science database on the A-Z list offers cited reference searching
Google Scholar provides citation counts in its results, click on the Cited by link at the foot of each result to see the citing articles
Publish or Perish uses Google Scholar data to search for articles citing your article of interest. See the ISSUE article (page 4) about this service that you can quickly download onto your computer desktop.