This toolbar was released by Alexa to celebrate 10 years of the world's most respected website ranking site. It offers four main indicators in your browser. 'Related Links' helps you find sites that are similar to the one you are currently visiting. 'Traffic Trend' is a 'sparkline' that shows the traffic trend from the last four months. Meanwhile 'Reach Meter' simply shows you the site's reach. And of course, there's 'Traffic Rank' which shows you the Alexa ranking of the site.
Previously, Firefox users were restricted to the StatusExtension toolbar which many users may continue to prefer anyway. StatusExtension provides all of this information plus more but it remains to be seen how long Alexa will continue to provide it with stats now that it has it's own version of the market.
If you haven't got the StatusExtension toolbar then this is definitely worth installing. However, if you've already got it, Sparky doesn't offer much more in terms of functionality and features.