A classic poultice herb with a long history of folk usage. Being in the Boraginaceae family it is a wonderful insect attractor and incredible food source for honey bees! 

A single comfrey flower is said to replenish its nectar every 2 hours, and this cold hardy perennial thrives in spite of total neglect.

Beyond that, the leaves of Symphytum Officinale are a wonderful garden fertilizer that can be added into compost tea and foliar feeds, or simply chopped down and spread around the garden as a green mulch.
 
Varieties:

Bocking 14: 

This is the preferred variety for gardeners who need comfrey with a self contained “clumping” habit. It grows in diameter each year but does not send out runners which makes this variety easier to control.
 
It has lovely blue flowers and produces a prolific amount of leaves for herbal preparations and garden mulching.

Hidcote:

This white flowered running variety serves as an excellent companion around young establishing trees that will be getting mowed around (mowing keeps the comfrey from taking over your yard).
 
In this way comfrey simultaneously suppresses weeds while pulling nutrients up from deep in the soil.