Clustermaps is a free online service that allows a website/blog to display the locations in the world from where its visitors hail. It is very popular and a cool service in my opinion. If you have a reasonable reader base, why not show-off :)). This service is a purely cosmetic add-on if you use Google Analytics to track your website traffic.
I do not recommend using this service if most of your website’s readers are from one small region or if your website is heavily loaded with scripts/flash as this thing will affect the page-load time of your site by a pretty good number.