LA ABSINTHE CLUB

Absinthe is a strong herbal liqueur distilled with a great number of flavorful herbs like anise, licorice, hyssop, veronica, fennel, lemon balm, angelica and wormwood (the flavor of anise and/or licorice, at least in contemporary forms of the liquor, tends to predominate).
Wormwood, the one that's gained the most notoriety, is Artemisia absinthum, an herb that grows wild in Europe and has been cultivated in the United States as well. Much of the liquor's legendary effect is due to its extremely high alcohol content, ranging from 50% to 75% (usually around 60%), plus the contribution of the various herbs. It has been assumed by many that the so-called "active ingredient" in absinthe is wormwood, although that is apparently not really the case.

It was traditionally served with ice water and a cube of sugar; the sugar cube was placed on a slotted "absinthe spoon", and the water was drizzled over the sugar into the glass of absinthe (typically in a 3:1 or 4:1 ratio). The sugar helped take the bitter edge from the absinthe, and when the water is drizzled into the the liquor it all turns milky greenish-white (the effect is called "louche").
The drink was referred to in France as "La Fée Verte", or The Green Fairy, which is a reference to its often dazzling green color (depending on the brand). The color usually came from the chlorophyll content of the herbs used in the distillation process; however, some disreputable manufacturers added toxic chemicals to produce both the green color and the louche (or clouding) effect that in reputable brands was caused by the precipitation of the essential oils of the herbs.
It is quite probable that the bad reputation absinthe developed was due to these low-grade and perhaps quite poisonous version of the real thing.

Wormwood had been used medicinally since the Middle Ages, primarily to exterminate tapeworm infestations while leaving the human host uninjured and even rejuvenated by the experience. At the end of the 18th century -- the age of revolution and skeptical humanism -- the herb developed a recreational vogue. People discovered they could get high off it. The problem was the means of delivery, as it was unacceptably bitter in taste.
A French expatriate living in Switzerland by the name of Dr. Ordinaire found the answer by inventing absinthe, which delivered both the herb and alcohol in a stunningly tart beverage, with a flavor resembling licorice. The most well-known maker of absinthe was French distiller Henri-Louis Pernod, who was impressed with Dr. Ordinaire's beverages and purchased the secrets of its distillation and manufacture.

Absinthe would eventually enjoy its greatest popularity in fin-de-siècle Paris, with Vincent Van Gogh, Paul Verlaine, Alfred Jarry and Oscar Wilde among its most ardent imbibers.

After its banning, imitations, using anise and other legal herbs in place of wormwood, appeared. The most well-known is contemporary Pernod, which was originally the best and most famous brand of absinthe; it's still made today but the similarity is only in color and brand-name. Pernod now has very little of the very floral, herbal content of the Pernod Fils Absinthe of old, is now a pastis with a strong anise flavor, and by all accounts is an entirely different beverage today.
In New Orleans, the preferred absinthe substitute is Herbsaint, a locally-made anise liquor which is used in cocktails as well as in cooking. It's an absolutely lovely-tasting pastis drink, at 90 proof, and has a flavor that I believe to be superior to Pernod. It's also used in making the superb cocktail called The Sazerac.
It's been asserted that the trouble the governments thought to see in absinthe wasn't due to the thujone at all, but simply to the alcohol -- I'd have to dispute that assertion. If it was just the alcohol, why have they still been making 120 proof Polish vodka all these years, and Bacardi 151, and Everclear? Clearly current regulations (which either ban or limit the amount of thujone content) seem to have a problem with thujone/wormwood derivatives. The bottom line is ... thujone is present in absinthe, but in such trace amounts that by the time you consumed a toxic dose you'd be dead of alcohol poisoning, many times over.
Apparently the distillation process removes most if not all of the toxicity of the wormwood in well-made absinthe; that, plus its trace amounts in the elixir, make absinthe -- consumed responsibly, as any strong spirit -- perfectly safe. Additionally, wormwood is also one of the herbs used (in trace amounts) to make that flavored wine and essential Martini ingredient that we all know as vermouth. The name of the drink comes from the German wermuth, which means wormwood.
Learned experts on the subject of absinthe also assert, after careful chemical study of the original recipes and processes, that the storied effects of thujone in absinthe are highly overrated. Any elusive "secondary effect" above and beyond the alcohol in absinthe is due to the multiple effects of the myriad herbs found in real absinthe -- some do this, some do that, some bring up, some bring down. It's a combination of the herbs that does it (whatever "it" is, if anything).
However, there are a lot of really stupid and misinformed people out there who think that wormwood is The Key to Everything, and seek pure essential oils of wormwood in an extremely misguided attempt to "get high". It has been shown that consuming pure essential oil of wormwood, a poisonous concentrate containing high amounts of thujone and with neurotoxins intact, can cause renal failure and death if consumed. Pure essential oil of wormwood is not absinthe. IT IS POISON.

If you plan to sample absinthe in reasonable, moderate quantities, I can recommend Kubler from Switzerland and St Georges' from California, as two legal in the United States brands with thujone.
I understand the best absinthe in the world is the elusive Swiss elixir usually called "La Bleue"; unfortunately it's still banned in Switzerland and therefore extremely difficult to get, and extremely expensive as well. Don't bother with the Czech Hills brand, although I understand Czech brand Sebor is quite good.