For the most part, Mac OS X does a good job at opening files and extensions in the proper applications. That is usually one of my biggest selling points to those considering the Mac. Just double click a file and the OS will take caer of the rest.
However, if you would like to make some changes, there is only so much you can do in the ?Get Info? window of Finder. RCDefaultApp is a ?preference pane that allows a user to set the default application used for various URL schemes, file extensions, file types, and MIME types.?