I have tried a few of the stats plug-ins, and the WP.com one is my favorite.
It is active on 2 of my 3 blogs, but I can't get it working on the third.
When I activate thr plug-in, I get the following message and link, highlighted in red:
WordPress.com Stats needs attention: please enter an API key or disable the plugin.
When I click the link, I am taken to the options page for wordpress.com stats, but there's no place to enter an api key.
I have gone to WordPress.com and have not been able to locate a place there to enter my api key either.