Twitter has started inviting a select group of users to test a new analytics product, Mashable has learned. Such a product has been rumored for some time, and a Twitter executive said earlier this year that Analytics would debut by the end of 2010.

With Twitter Analytics, users will be able to see a plethora of data about their account; for example, information about which tweets are most successful, which tweets caused people to unfollow, and who the most influential users are that reply and retweet their messages.

In the images below, you can see how some of this data is charted. A bar graph over 6-hour increments shows you mentions, follows and unfollows, for example, and you can filter tweets by categories that are defined as "best," "good," and "all."

In what will no doubt come as a blow to the third-party vendors that are already providing similar tools, we’re hearing that the product will be free and that Twitter has no immediate plans to charge.

We’re working to gather more information, but we may hear more about it later this afternoon, when Twitter’s Evan Williams takes the stage at the Web 2.0 Summit in San Francisco. Stay tuned.

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[thanks @mongoosemetrics for the images]