Alexa is a company that tracks the Internet. They make a browser toolbar and constantly crawl the web for statistics. Their graphs are well-known, showing statistics like the popularity of various websites over time.
Alexaholic is a service that lets you “mash up” two or more website results from Alexa. Compare, for example, Google, Yahoo, and MSN over time. Looks pretty good for Google! In fact, if your site is in the top 100,000 on Alexa, you can compare yourself to similar sites and see how you’re doing.