Over the last couple of months I found some great articles on different topics that every web developer or designer should have read. Many of the subjects are not really new or ground breaking, but they cover a lot of basic stuff (sometimes we tend to skip those parts in the initial learning process) as well as in-depth information. I’m sure the next website or application you’ll build will benefit from it. ;-)

John Kleijn wrote a great series of articles on Design Patterns and OOP PHP on phpfreaks.com:
http://www.phpfreaks.com/profile/448191

On Perishable Press there is an extensive article on .htaccess tips & tricks:
http://perishablepress.com/press/2006/01/10/stupid-htaccess-tricks/

This article on Yahoo on how to optimize website performance is great value as well:
http://developer.yahoo.com/performance/rules.html

Maybe not so much for freelance web developers or designers but more for people responsible for a corporate website or digital department (however still an interesting read):
http://www.smashingmagazine.com/2009/02/10/10-harsh-truths-about-corporate-websites/

And to end with, a sum up of rules web designers should consider when designing for web:
http://www.littleidiot.be/2008/01/04/designing-for-web-some-rules-to-consider/
(yes, indeed, I wrote that one myself based on my experience of working with designers)

I will try to extend this list in the future, or if you have any suggestions, let me know.