Jacob Nielsen, reknowned usability consultant, has done extensive research on web site usability and his articles on Writing for the web is a great resource for anyone who wants to improve their web site copy and content.

The essays and reports on writing for the web include:

• How users read on the Web [read this one first]
• How little do users read? — users spend 4.4 seconds for every extra 100 words on a page
• F-shaped pattern for reading web content, as seen in eyetracking studies
• Eyetracking of people reading email newsletters
• Low-literacy users exhibit different behaviors
• PR and press releases on corporate websites (75 design guidelines based on usability studies of how journalists visit company sites)
• Writing style for print vs. Web
• Blah-blah text: Keep, cut, or kill?
• Email newsletters (165 design guidelines: scannability even more important than for websites)
• Writing transactional email and confirmation messages
• Teenagers on the Web: poor reading skills and low patience levels mean that text has to be ultra-concise for teens and that more information must be communicated in images
• Tagline blues: what's the site about?
• Passive voice is redeemed for Web headings
• Use old keywords when writing to be found by search users
• Show numbers as numerals when writing for online readers
• Microcontent: writing headlines, page titles, and email subject lines
• Company name first in microcontent? Sometimes!
• Long vs. short articles as content strategy
• how to write inverted pyramids in cyberspace
• Information pollution
• Intranet usability, including guidelines for intranet content, news on intranets, HR manuals, and how to present information about projects, teams, and individuals on intranets
• Full paper documenting the original research from 1997 (long): Concise, SCANNABLE, and Objective: How to Write for the Web (unfortunately this paper was written for print and not online)
• Case study: Applying Writing Guidelines to Web Pages improved usability by 159% when rewriting sample pages from a popular website