Design subscriptions.

http://www.metropolismag.com
My favorite of all three. This magazine advertises itself as "architecture < culture > design" and includes many disciplines. With a subscription you gain access to past articles, which is a valuable resource. Design education is often discussed as well.

http://www.dwellmag.com
An alternative approach to residential design.

http://www.commarts.com
A showcase of juried design competitions in graphic, photography and illustration. Great articles and a good resource for keeping a finger on the current state of the profession.

http://www.emigre.com
A typography house. The articles are a bit too post-modern for me, but their typefaces are lovely.

http://www.houseindustries.com
Another great type house that demonstrates how branding can propel an idea. Not really a magazine but a good catalogue.

http://www.aiga.org
The American Institute of Graphic Artists. THE association for graphic designers in the U.S. The forums are interesting and cover a broad array of topics.