Update: 2014 version now out!
I made a Catholic Holy Days of Obligation calendar for the year 2013, which you can download at:
Catholic Holy Days of Obligation calendar for 2013 (.ICS file, suitable for importing into iCal or Outlook).
It’s based on About.com’s list of Holy Days of Obligation by Scott P. Richert. This year, it also includes embedded links, so you can read more about each Holy Day. If you find any errors, please let me know.