Edward Tufte's books are required reading for anyone who presents data and information in graphic formats.
What is web site usability? Jakob Nielsen offers many suggestions for making the web more useful and easier to use.
Lynda Weinman's site offers great training materials for design and creative media software (Mac/PC).
Jim Heid offers hidden tips and tricks for iTunes, iPhoto, iMovie, iDVD and GarageBand on his web site. He has written several books, including the Macintosh iLife '05 book and DVD set.
A site full of the finer points of page and site building for anyone with an interest in web development technologies.
In-depth, frequently updated info on executing complex web code.
A lot of information about graphic design.