A Historical Calendar Applet
I wrote this applet back in 1998 to answer a question that was bugging me while I was plotting out a historical novel (which perhaps I'll finish some day, if I ever have time): what would the calendar have looked like at a particular date in the distant past? This isn't just a matter of calculating Easter and the feasts like Pentecost that depend on it. Saints' days change over time as new ones come along and old ones fall out of favor or are moved. Also, different regions may celebrate the same saint on a different day. Rather than simply look up things for the particular place and time I was dealing with, I decided to create a general-purpose solution.
This applet works from a database of about 450 feast days, and will place them on the appropriate day and month depending on the year/region you select. In particular, it contains information about all the Vatican II revisions to the calendar, so for earlier dates, you will see the saints as they were celebrated traditionally. The default settings will automatically exclude saints that weren't celebrated in a particular year (e.g., because they hadn't been born yet).
If you need additional help using the applet, you can read the instructions. Before you send me any comments, you might also want to read the implementation notes.