The KING of all copywriting links
http://www.useit.com/papers/webwriting/
This is the most useful collection of articles on the topic of copywriting that you will ever find. You MUST read. They are by the world renowned usability expert Jakob Nielsen - there is no higher authority than this. A couple of the articles requre payment (just skip them unless you have a lazy $500 to burn).
Includes links to other useful articles such as:
http://www.e-gineer.com/v1/articles/web-writing-for-many-interest-levels.htm

Importance of persuasive copy
http://www.clickz.com/experts/crm/traffic/article.php/3412131 - this is gold!
& there's loads of great articles on this site.

SEO / copywriting / programming tips
http://www.netmechanic.com/news/index.shtml

Web Writing
http://alistapart.com/topics/content/writing/

Articles & tools for web authoring
http://www.selfseo.com/copywriting_articles.php
Also see the large number of other web-related article categories, and online tools in the left margin.

Marketing focus
http://www.clickz.com/showPage.html?page=experts

Good Introduction with links
http://webdesignfromscratch.com/writing_for_the_web.cfm