traffik plugin

Google Analytics is the one I use. You can set it up easily with the Google Analytics for WordPress plugin.

http://wordpress.org/extend/plugins/google-analytics-for-wordpress/

Another one that I haven’t used but has a good reputation is WP-Stats, which will have the stats showing in your site’s back-end (as opposed to Google Analytics where you have to log into Analytics to check your traffic):

http://wordpress.org/extend/plugins/wp-stats/

Cheers,
Eric