These databases give the number of times an article has been cited (as seen by those databases)
Publish or Perish
Software to mine impact information from Google Scholar and Microsoft Academic Search
• Read What Publish or Perish is for to understand its potential uses
Requires installing software; both Windows and Mac (through Windows emulators) supported
Web of Knowledge databases:
Medline
Web of Science
BIOSIS Citation Index
Google Scholar (via the Fresno State Library proxy)
The number of times an article has been cited is often shown on the publisher's site (for example Wiley or ScienceDirect/Elsevier).
SPARC on Article Level Metrics