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Terraism, I used to own some of the older Greyhawk products.....I stupidly sold them to my old DM for a quick fix of cash, but whatver.......
Anyway, the products I really liked when I had them were:
1983's World of Greyhawk Boxed Set
A1-4 The Slave Lords...this was a string of cool adventures that I think you can still find on Ebay or elsewhere.
The original Temple of Elemental Evil....this book was more toned down on earth shattering importance (anything with Tharizdun tends to jack up the CR level.....). It detailed Hommlet when it was much smaller, and the PCs only have to deal with Iuz and Zuggtmoy.
The Cult of the Reptile God....the first GH adventure I ever ran. This IMO was a really cool adventure that was set near Hochoch in the Sheldomar Valley. Its pretty tightly written, and its got a lots cool stuff in it.
Those are the few 1st edition titles I was lucky enough to have (at one time). Some others are now free at TSR's websites. If I remember correctly the modules were the Secrets of Bone Hill, Palace of the Silver Princess, and possibly the old version of the Tomb of Horrors (this might not be on anymore).
Another favorite of mine is not specifically Greyhawk, but it definetely has its roots there. Its the Rod of Seven Parts boxed set. This high level adventure was one of the best written adventures I've seen. Its up there with Monte Cook's Return to the Temple of Elemental Evil.
Hope that helped,
C.I.D.