Natto. Fermented soybeans. Very healthy, yet the sort of food that will make the unprepared run screaming into the hills. It's the slimy surface. Natto might be an acquired taste, but maybe it's more a food that you need to grow up with. It's the Japanese equivalent of lutefisk.
I had natto once, served with rice. The sliminess was difficult to deal with, of course, but apart from that, it wasn't bad. I keep telling myself I need to try natto again, but I haven't quite gotten around to it.
Natto has become the centerpiece of a great food scandal in Japan, because excessive health benefits were claimed for the slimy soybeans. Maki has a good post summing up the situation at i was just really very hungry.