Making your own bitters!
This is an absolutely fantastic guide, but I decided to also do some further research:
http://www.adventures-in-cooking.com/2012/11/diy-bitters-part-i.html
An even more comprehensive guide:
http://spiritsandcocktails.wordpress.com/2008/04/22/how-to-make-bitters/
Agnostura bitters are evidently primarily flavored with gentian, a bitter herb I have never heard of before.
Lists from old timey times have listed wormwood leaves, goldenseal rhizome, blesses thistle leaves, yarrow flowers, angelica, and bitter orange peel as other common bitters ingredients.
As I have learned, there are three basic components: the bitter herb, the flavor, the base. Or, as Mountain Rose puts it: Essentially, bitters contain three flavor components: a dried bitter
root or bark, dried fruits or vegetables, and dried spices or herbs.
List of bitter herbs:
Wormwood, quassia bark, gentian root, fenugreek, yarrow flowers, orris root, dandelion leaves,
Mildly bitter:
Chamomile, mint, chicory, horehound, rue.
List from website of bitter blend for cherry bitters: rose petal, burdock, milk thistle, dandelion, apple, barberry, fennel, fringe tree.
Fruit stuffs:
Lemon, lime, orange, grapefruit, pear, apple, blueberry, cranberry, celery, chiles,
Some nice spices:
Clove, cinnamon, vanilla, allspice, star anise, peppercorn, ginger, nutmeg, cardamom,