If you're logging in W3C format in a text file then you can run AWStats or similar over it.
Text File Logging
You can save ISA Server logs to a text file, in one of the following
World Wide Web Consortium (W3C) format. W3C logs contain both data and directives, describing the version, date, and logged fields.
Because the fields are described in the file, unselected fields are
not logged. The tab character is used as a delimiter. Date and time
are in Coordinated Universal Time (UTC).
ISA Server format. ISA Server format contains only data with no directives. All fields are always logged. Unselected fields are logged
with a dash, to indicate that they are empty. The comma character is
used as a delimiter. The date and time fields are in local time as
configured on the computer.
By default, log files are saved in the %ProgramFiles%\Microsoft ISA