I really like http://whos.amung.us
It shows real time data on who's on your site\blog right now by simply using a little gif file on each page (in the blog template, for example.
here are the real time stats for my blog:
http://whos.amung.us/popular/2baujzs9
plus you can see a little gif image on the left of each page that shows how many people are reading this page right now..
very nice!
[via Amit]