It pops a cool little Alexa status bar in the bottom-right of the browser. It’s a nice way to get a quick feel for how popular a site is (if you trust Alexa, that is). I wonder if Compete is going to release a tool like this soon? (Their toolbar is the most annoying thing ever.)
Here’s a quick overview of Sparky (stolen from Alexa’s site):
Sparky’s trendline feature allows you to track a website’s popularity over time, plotting the traffic rank for the last 4 months on a handy little sparkline.
Reach Meter and Traffic Rank
Sparky’s reach and rank features give you a quick overview of a site’s popularity. The blue bar gives you an overall indicator – is this site popular or not? The rank number tells you exactly how this site ranks in comparison to all other sites on the Web.
Sparky also comes with Alexa’s famous patented Related Links feature built right into the browser. Now, no matter what site you are on, Sparky can help you find similar sites.
With a click on Sparky you have immediate access to a wealth of additional information. Do a search for the site, find sites that link to it, see who owns it, where it is located, related links and much much more.