Forum OpenACS Development: Re: Get number of visitors currently browsing the site
For logged in users, yes definitely, see these threads from Oct. and March 2003. I think there was also some more recent discussion of that stuff just in the last week or two, but I can't find it right now.
For non-logged-in users, dunno, the above might track number of user_id 0 hits, or it might not. (Try it and see.) If not, what you want is to instead look in the AOLserver access log. It is easy to configure AOLserver and OpenACS to log the OpenACS user_id to the AOLserver access log, so to check unregistered users you can just grep for only user_id 0 hits in the log.
Keep in mind that the web pages showing the "who's online in the last 10 minutes" sort of info are intended for transient human use, not for any sort of load tracking over time. If you want to graph load or track which users have been clicking on what over time (clickstream data warehousing, etc.), then post-processing the AOLserver access log is the right way to go.