you could simply write out a CSV file (importable into Excel) as a piece of content in the Repository with JavaScript, i.e. using the JavaScript Console. I don't know of a specific, existing tool that already does this…
When using LDAP or another external directory, this depends on what your synchronization settings are. You can always write out all the users with user ID, first and last name (if set in / retrieved from the directory), since Alfresco synchronizes with the directory or lazily-creates the user upon first login. Additional detail information is configuration specific.