Find a Catholic Mass
This Google Maps mashup will let you find a catholic church seemingly anywhere in the world. For many of those churches Find a Catholic Mass can also tell you the times that mass will be held.
To use the map just type a location into the search box and the results are returned on the map. Clicking on any of the map markers opens an information window with the name and address of the church and the times of mass.
The site says that a wiki is being added to the map soon. Presumably this means that users will be able to add mass times to churches that aren't currently in the site's database.