On the Alexa site, you can click the Traffic rankings tab to see an ordered list of the top 500 Sites updated daily.
The Movers and Shakers list is also interesting. It is a snapshot of the "right here and now" on the Web, and is useful for aligning your SEO efforts with Web-wide trends in real time.
It is worth spending time learning about popularity on the Web if you want to build successful sites. Alexa provides the tools you can use to see for yourself what is trafficked, and what is gaining or losing among top-ranked sites.
You can also use Alexa to see traffic statistics for sites that are not in the top 500. For almost any site that has been around a while, Alexa will give you an idea of traffic statistics, and whether it is gaining or losing traffic.
Alexa lets you enter descriptive information about your web site, which others can see if they check your site traffic using Alexa. You can also make sure that Alexa provides a snapshot of your home page along with its statistics. Since this service is free, it is certainly worth entering a site description and monitoring your Alexagarnered statistics.
Alexa works by collating results from users throughout the Web who have installed the special Alexa Toolbar. (If you'd like, you can install the Alexa Toolbar and help with popularity statistics.) There's some question about the statistical validity of Alexa for less-trafficked sites because of this method of gathering dataAlexa's results are probably skewed towards users who are already web savvy and heavy users.
Most likely, Alexa's results are not very meaningful for sites that are ranked below 100,000 in popularity (very roughly, with fewer than 10,000 visitors per week).