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As with getting a list of tags, this is not implemented in the most current version of CVS, but it's highly likely that it will be implemented soon. I imagine the command will be called cvs list with a short form of cvs ls, and it probably will both parse the modules file and list the repository subdirectories.
In the meantime, examining the CVSROOT/modules file (either directly or by running cvs checkout -c) is probably your best bet. However, if no one has explicitly made a module for a particular project, it won't show up there.