I'd like to be able to submit a DoI or PubMed ID and find all the papers that cited that paper.
In other words, I need a publicly-available citation graph database that covers papers in PubMed.
The bottom comment on this question refers to a publicly-available citation graph: How do I get a citation dataset with full text articles in PDF format? but does not give any details of where it can be found.
Does anyone know of any such citation databases?
UPDATE: To be clear, I really need a database - not a service like Google Scholar - because I want to do it programmatically for many hundreds of thousands of articles.