AIGA: Graphic Design Theory - AIGA has a great article on the theory of graphic design, which is an essential read for anyone trying to delve into the concepts behind the practical aspects of graphic design.
Core 77 - A general design webpage, Core 77 helps designers build their skills in multiple areas. This page is a guide for graphic designers with tools to help them hone their eye for typography.
Dieline - This site focuses exclusively on packaging design, campaigns, and trends, featuring editorials, projects, and design competitions.
Graphic Design History - This series of essays was written by professor Nancy Stock-Allen, who approached the subject by topic rather than as a timeline.
Munsell Color Blog - In the spirit of Munsell’s passion for making sense of color, the Munsell color blog was created to continue Munsell’s legacy of color expression in art, industry, and education.
The One Club - This American non-profit organization produces four annual award competitions: One Show, One Show Design, One Show Interactive, and One Show Entertainment.