You give it author and title, it gives you other bibliographic details including a DOI (the part I was looking for; basically a more-permanent version of the publisher's official web address for the paper). The link is to their "guest query" page; they also have a different interface for batch queries for registered users, but I think that's not intended for individuals.


A lot of journal sites are using it to provide links in the bibliographies of the online versions of their articles. It's quite a handy thing, IMO.
I have occasionally found papers with DOIs that it didn't know about, and of course many of the more obscure journals don't even provide DOIs, but its database seems pretty comprehensive.
Should be pretty comprehensive, indeed; it's "the official DOI link registration agency for scholarly and professional publications," according to their website, which means that if a paper has a DOI that Crossref doesn't know about, nobody knows about it other than the publisher. :) (Or possibly because there was an error somewhere and the DOI printed on the paper is not the DOI actually assigned to the paper.)