Cave fungus (Fallout 3)

 is a food item found in Fallout 3. It can restore some of your Hit Points while slightly reducing your radiation.

It can be acquired from Little Lamplight by striking a deal with the "cook" Eclair or the "Doctor" Lucy for either strange meat or buffout respectively. Either way, the deal has to be honored through Mayor MacCready. Alternatively, it can be acquired directly through Mayor MacCready after a successful speech challenge but this will result in negative karma. The price is lowered with a higher Barter skill or with the Child at Heart perk. This arrangement can be extremely profitable for you due to the high value of cave fungus. Some cave fungus can also be located in the cafeteria area of Little Lamplight and taken with no negative karma. If you decided to take the 'good' path in Andale and kill all of the cannibals, you'll be able to loot a lot of strange meat from there, which would allow you to save Buffout.

According to the residents of Little Lamplight and its history, the fungus appears to "like" strange meat. This is connected to the fact that the original fungus grew "from the water where the first Little Lamplighters dumped the bodies of the mungos". Once you learn that strange meat is actually human meat, all the pieces fit together.

The fungus' radiation scrubbing ability makes it an effective and nutritious health booster out in the wasteland and shines some light on a potential hope for human survival on a greater scale. Of course, a fungus that consumes human flesh may not be the best hope for humanity.

Unused Vault 106 note
The following is from a cut Vault 106 note in Fallout 3. It describes fungus growing in caves.