Visitors Online is a free Magento extension that alows store owners to add a block indicating number of visitors on the site.

To embed the extension on the site one only needs to insert the block through CMS – visitors online information would appear immediately. Let your shoppers know they are not alone

This is a free extension – just login and download it.

Visit product page – http://ecommerce.aheadworks.com/free-stuff/visitors-online.html