This also goes for user-installed fonts, screen savers, pref panes, startup items, etc. -- put them in the appropriate directories in /Library not /System/Library.
For these things, you can even put them in ~/Library (that is inside of your home directory, for example /Users/YourName/Library). Most Mac applications will search there, too. Does not work for Perl modules, though (unless you manually adjust @INC, which of course is easy), and other users cannot see or use them (which in many cases is a good thing).