I've definitely rolled up multiple dungeons ripped straight from Diablo 1 and 2, including the big guy himself, but that's fairly standard stuff
I ripped the entire premise of a campaign from a book series by Karen Miller (The Godspeaker Trilogy). From one of my old DMs I've imported in Gnome behaviors (insane made geniuses who can't do anything without explosions... although thats from Dragonlance too
) One of the more humorous things I've dragged from an old DM was making all the magic merchants pixies. I had the players find the Zerg in my last campaign, but after the only character interested in fighting them wandered off from the party that storyline died kinda quick
The campaign I'm building right now rips shamelessly from the Heroes of Might and Magic series, especially the first 3 games. I've used the final battle scene between Armageddons Blade and the Blade of Frost before too.