CiteSeer is a citation indexing tool concentrating on computer science, produced by PennState.
It is online at http://citeseerx.ist.psu.edu/
Material is added to CiteSeer by a combination of human article submission and collection. Authors can create profiles for themselves, but you will want to do a document search-- make sure the Documents tab in the upper left is highlighted, then search for your author and/or title.
In the results that come up, you will see which ones have citations to them in the bibliographies of articles in CiteSeer.: Cited by [x] ([y] self) 
Here we see that the 3rd article has been cited by two articles in Citeseer. The Self number tells you how many times the authors of that article cited it in their other papers.
A useful feature here is that you can also try your search in other indexes that include citation measures, using the links in the right column.
To see the list of articles that cite the article you are looking at, click on Cited By [#] 
Citeseer generally includes the full text of the article as a PDF, so to see any article, click on the title.* Thenn click on the Cached icon in the upper right to download the paper.
Confusingly, the entries for particular papers use the term "Citations" which actually refers to a list of the references in the bibliography of the paper, not to items citing the paper.
Go back up to Citations: # under the title to see how many times the item has been cited in Citeseer.