Alexa is a company that gathers information about the internet and the relative popularity of websites. Alexa Toolbar is a tool that serves two purposes - it provides you, the user, with information about the websites you are viewing, while providing Alexa with information about what websites are hot right now.
The Alexa Toolbar works with Internet Explorer. It doesn't get off to a great start - the toolbar isn't very attractive and it's a little slow, but not enough to be a real problem. Alexa Toolbar will appeal to users who are interested in knowing how popular their or others' websites are. It provides direct links to the hottest websites, search terms and direct links to Alexa data about the webpage you're viewing.
The app also links to archives - old versions of websites - and allows you to tweet information directly. You can also use Alexa Toolbar as a regular search engine and take advantage of integrated search services such as Wikipedia and Amazon.com. As toolbars go, Alexa Toolbar isn't that impressive. Obviously if you are a frequent user of Alexa, it might provide the speedy link to the information you need, but unless you are already a regular user, you're unlikely to fall in love.
Alexa Toolbar provides rapid access to Alexa information but, as a toolbar, isn't very impressive.