I personally haven't done anything really crazy (pumpkin, but that's old hat around here). However, I have drank several beers with strange mash/boil additions, including:
Pies, both in the mash and as a "pie hop."
Doughnuts (mash)
Peanuts (mash) - apparently this is the best way to get the peanut flavor in the beer, and it was a damn good PB&J beer.
Shellfish (boil, I'm assuming) - kind of like an oyster stout, but using a variety of shellfish. Had this at EBF last year - forget the name of the brewery. Kinda weird...
And of course the HBT folks here have mashed/boiled all sorts of things. Anything with convertible starches or sugars can be fermented, and most of it has been...