PMC (PubMed Central) has just released an upgrade to the ID converter:
This utility allows you to start with the unique identifier for an article that is in PMC, and find additional unique identifiers that may apply to the article. Further details here.
Improvements include support for DOIs, auto-detection of the ID type based on its format, and enhanced output. It also provides output in any of several different formats: HTML, XML, JSON, or CSV.
This tool uses an underlying web service, that is also publicly available for those needing programmatic access to this data. ID Converter API documentation here.