I have never met a Gentian that I didn't fall in love with at first sight, and Gentiana acaulis is an example of why.  This diminutive Gentian, native to southern Europe, grows to less than six inches and blooms in late spring/early summer with luscious, inky blue flowers.  Gentiana acaulis prefers full sun, and it is drought tolerant and, although a slow spreader, after a time it can be easily divided.  Full sun. Zones 3-9. (Illustration by William Curtis from The Botanical Magazine, Vol. 2, 1788.  It is from Wiki Commons and in the public domain.)

Gentiana lutea, native to the mountains of central and Southern Europe is a yellow-flowering Gentian with a most interesting past and present.  Its roots were used during the Middle Ages as an antidote to poison, as well as for treating fevers and intestinal worms, to name just a few of its many uses.  Today it is a brewing agent and is one of the components of Angostura bitters, vermouth, and the New England drink, "Moxie."  Gentiana lutea is often called Bitter Gentian because one of its bitter properties is the one by which all bitterness is judged.  Have you ever heard someone say that something is as "bitter as gall"?  Well, this Gentian is also known as Gall Weed.  You can read more about this interesting plant here.  Gentiana lutea is slow to get started and doesn't usually bloom until its third year, but - get this - it can live up to 50 years, which I find difficult to fathom.  The blue/green, ribbed leaves are about a foot long, and the plant grows to about three feet, so give it room - after all, it will need it for a long time.  The pale yellow flowers are in tiers, each flower measuring about an inch across.  Sun/part shade, Zones 3 - 8. Quarts.  (Illustration by Franz Eugen Kohler from Kohler's Medizinel Pflanzen, 1879, from Wiki Commons and in the public domain)