eMarketer
This site offers statistics and commentary about Internet trends, as well as practical marketing advice for e-businesses.

Forrester Research
Forrester is an independent research firm that focuses on the future of technological change and its impact on businesses, consumers, and society. If you're researching a particular Internet market, there's likely to be a Forrester report with relevant data. A subscription to all its reports is pricey, but many briefings and reports are available for free on the website.

Jupiter Communications
This research firm provides clients with views on industry trends, forecasts, and best practices. Jupiter's research and advisory services, offered on a subscription basis, provide Internet business reports and data.

Media Metrix
This site ranks websites by the number of unique visitors, reach, and other measures. You can view a list of the top 50 web properties ranked by the number of monthly unique visitors for free. If you want more detailed information, there's a hefty subscription fee.

PC Data Online
Similar to Media Metrix, PC Data Online provides website rankings and demographic data about website visitors. You can view a report showing the reach and number of unique visitors for the top 50 websites for free (click on Public Reports). To see more detailed information, you'll need to pay a subscription fee.