Clover

Clover is a compost crop that will suppress weeds and attract beneficial insects to the garden.

There are a many different clovers with different strengths: production of nitrogen, hardiness, cold resistance, beauty.

Environment
Ground cover plant that needs an inoculant for root bacteria to grow and produce nitrogen.
Growing environment is dependant upon the type of clover - a number of different types can be found here.
Clover is reported to be not effective on sandy soils.

Clover examples
Berseem Clover produces a great amount of nitrogen but is not cold hardy[1] Oats have been shown to grow well as a companion plant.
White clover is the most common and vigorous growers. It is cold resistant and can be used to create lawns that are great for insects and bees.
Alsike is a hardy, winter resistant plant.
Red clover is considered to be the best for compost cropping.

Companions
Fruit trees - to attract beneficial insects to combat pests[2]

Sources and Info
[1] Berseem clover overview
[2] Clover as a companion plant
BBCGardener Red Clover