A free DOI lookup tool is available at:
http://www.crossref.org/guestquery/
Alternately, if you need the DOI of a journal article that's indexed in PubMed, try using PubMed to find the needed number:
- Find the PubMed record for the article (PubMed's single citation matcher may be helpful).
- Use the PubMed "Display setting" drop-down menu to select and apply the "PubMed" format.
- Look for the [AID] fields near the bottom of the PubMed record. The DOI if present, will be followed by [doi]