Pepper

Black pepper is a supplement as well as a spice.  It is a rich source of magnesium, and also supplies vitamin K and iron.
An excerpt from http://en.wikipedia.org/wiki/Black_pepper :
It has been shown that piperine can dramatically increase absorption of selenium, vitamin B and beta-carotene as well as other nutrients.

And from http://www.whfoods.com/genpage.php?tname=foodspice&dbid=74 :
Black pepper has [...]

Avocado

Here is a nice paper about the avocado:
 http://www.avocadosource.com/CAS_Yearbooks/CAS_76_1992/CAS_1992_123.htm
Abstract. Wrong ideas about the avocado are reducing its consumption. Avocados have high nutrient density-per calorie, they are rich in important human nutrients. This, with their giving a feeling of fullness, plus their palatability that reduces the urge to over- indulge in excess sugars and saturated [...]