Is it possible to see what AD users are currently browsing an internal (intranet) IIS web server?
You have an understanding issue with the HTTP protocol. It's not like, say, FTP, where the client computer is keeping a persistent connection open to the server computer while the user is "browsing". Connections from clients are transient.
Having said that, assuming you're requiring authentication to the site, you could use the site's log file to see who has recently been accessing pages. If you're not requiring authentication, you're just going to see client IP addresses.
I don't think you can get the individual users at the server level, but you can user perfmon counters to get counts of how many active connections there are.
For IIS6 (from the Technet Article):
1.Open Administrative Tools, and then click Performance.
2.Right-click the System Monitor details pane, and then click Add Counters.
3.Do one of the following:
To monitor any computer on which the monitoring console is run, click Use local computer counters.
To monitor a specific computer, regardless of where the monitoring console is run, click Select counters from computer, and specify a computer name or IP address.
4.In Performance object, click:
Web Service to monitor active Web connections.
FTP Service to monitor active FTP connections.
5.Click Select counters from list, and select Current Connections.
6.Click All instances.
7.Click Add, and then click Close.
See Evan's answer. Also netstat | find ":http" will show you which hosts connected in the last minute or so (whatever the TIME_WAIT interval is).
The problem comes hard as the definition of "currently" varies.
If actually you need to know who accessed this server in the last 10 or 1 minutes, you can always take a look at Security event log to check logon events. But that's only a rough estimate.