Many people run Zope and Apache together. If so, apache's access.log usually gives more detailed information than zope's Z2.log; eg ip addresses (or domain names) of visitors. It also tells you what visitors are trying to do, rather than what zope is actually doing. Eg if zope is busy or hung, requests will appear here but not in Z2.log.
On debian GNU/linux this is at /var/log/apache/access.log.