This is a sample mongodb db config file. I created it when setting up mongodb 1.4 in 2010 in a format similar to Apache config files (explicit commented defaults).
The file is old and out of date so it will need to be updated with new options and retested. Hopefully I can get an updated version out soon, perhaps even one for each version of mongodb.
Most of the options were culled from the mongodb docs, although some defaults are my own.