The D&D adventure modules from the Basic, Expert, Companion, Master and Immortals (BECMI) rules.

Most though not all of these adventures, now considered Old School, I guess, were set in the Mystara campaign setting and published by TSR in the 1980s.

They comprise:
B Series (B1-12, BSOLO)
X Series (X1-13, XL1, XSOLO, XS2)
CM Series (CM1-9)
M Series (M1-5, MSOL1-2)
IM Series (IM1-3)
DDA Series (DDA1-4)
O Series (O1-2)

Some of the AC (Accessory) Series included mini adventures on Mystara, but these are not listed here, and neither are the Hollow World GAZ series of Gazetteers which I have listed separately.

I recommend mystara.thorfmaps.com for other Mystara material.