Though there are many analytics plugins for WordPress, we recommend Google Analytics for WordPress by Yoast.
This plugin easily allows you to add advanced features to your Google Analytics tracking:
- Easy authentication using Google Analytics API
- Custom Variables with a choice of tracking Logged in users, post type, author, tags, publication year and categories
- Built in Event tracking of downloads, email addresses and external links.
- Easy implementation of sub and cross domain tracking
- Ability to tag campaigns with # rather than ?
and is regularly updated by the author.