Well I fiddled arround and I sort of made a simple script to show who is / was online the past .... minutes.
How ever there is 1 (bigger) bug to get rid of, I use the COOKIES stored by WordPress and filter out the author name. But I reckon 10 - 15% of the time that is a wrong cookie / information. So if anyone knows how to get that info it would be appreciated..
The script can be downloaded (with README.txt) at http://www.mcmike.nl/whoisonline.zip
Unzip and read the README.txt or mail me @ email@example.com :)
Have fun :)