I've recently become privy to the idea that Dragon Magazine has some pretty awesome stuff in it, particularly relating to Planescape. I might be wrong, as I've never actually picked up an issue, but from what I've read about it, even on these forums, it seems to take up what WotC decides is too ungeneric for its purposes.
That being said, before I start ordering issues online or living in my local used book store, I was hoping that some of you more experienced Dragon readers might recommend some good issues. They seem to be in the 300's, from what I gathered through a short google search, and I have no clue where 3e even starts, and if its worth going earlier than it. Plus, I don't think I can cough up the dough to buy an entire library, so if you could name a few of your favorites, that would be really great.
If you're unsure of where to start you might want to pick up the Dragon Compendium, a hardcover book that collects some of the best Dragon articles.
Personally I really like the Demonomicon of Iggwilv articles which go into greater detail about the demon lords than even the Fiendish Codex.
There was also an article on spells of the Demonomicon in Dragon #336. That particular article tied into the series by talking about the book itself and some of the unique spells therein. The first one to be made using advancement rules from the FCI was Kostchtie. All the rest from now on should be built that way.
Also, the Ebon Maw had a very Demonomicon style article in Dragon #312 which had him statted out along with some of the info that is typical for a Demonomicon article, although the first "true" Demonomicon article was Pazuzu in #329.
#330 had some great Far Realm stuff.
#351 had an expanded World Serpent Inn and cool locations for 7 classic campaign settings, including Planescape and Ravenloft.
I only started subscribing to Dragon myself a little while ago so I'm not familiar with any of the old stuff.