I've probably got one of the worst griefer (I didn't even know they had a name) stories, and mine too involved Diablo 2. Ours was actually a team of 3, 2 of us that actually enjoyed D2 and 1 that simply was good at loading bots. We scrapped together a quick 5 minute clan page, and started flooding all the bnet channels with messages of free high level items for joining our guild. We had an application form where you had to list all your accounts that you had. We also said we'd soon be adding forums to the page, and that you would need to pre-register a forum account/password. Needless to say, a lot of the times, the forum password was the same password to D2 accounts. I think by the time our scheme had gotten caught onto and others were emulating it, we had nearly 8 accounts full of mules holding all the items we had horded from our dirty trick. We didn't even keep the accounts, just stripped them of their value and then changed the password back. I can remember getting messages from the people on new accounts or friends' accounts pleading to leave their things alone. Still feel really bad for having done that. I remember I used to tell the story a few years ago, and end it with saying I'd be going to hell for that. Probably a fair assessment.
And for what it's worth, a few months later, I felt so bad about it, that I went into random games, and emptied the accounts of all their worth. Of course, then my friends hated me.