M series (purple): this stands for "Maze" after Maze of Zayene. All were written by Rob Kuntz and originally released through his small press outfit in 1987 or thereabouts. This was initially conceived as a four-parter, but a fallout between Rob and Necromancer occured, leadind to its cancellation after part 3. They are all "high level" (in 1e terms - this means 8th to 12th) and draw mixed reaction, especially from the more 3e-y crowd. I'd recommend that you check out parts one and two - these are pretty good after a few alterations.
Melan,
I have to disagree with you. I am old school D&D. I cut my teeth on the white box set (but I am a fan of 3e). But the M series were really bad. I believe they are some of the worst explains of what was wrong with earlier versions of D&D.