I think that non-addictive substances, and possibly even addictive substances which are currently illegal should be legalized. After all, Alcohol, Tobacco and Caffeine are already legal.
In many states, marijuana is the largest cash crop. The US does a huge trade in drugs every year, and we are losing all of that potential tax money! If we legalized drugs, and taxed the heck out of them, then many of us, especially the poorer segments of society, may not need to pay taxes at all anymore. (Unless they used drugs, of course.)

Many would argue that if drugs are legalized, then people would use them. It's a lame argument, and current drug usage levels prove it to be untrue. People use drugs whether they are legal or not. Many people in our fragmented society use drugs for a variety of reasons. Making the drugs illegal does not help these people. Dealing with the causes or roots of the individual drives and societal conditions that precipitate drug addiction works much better than writing and enforcing laws.

In The Netherlands], many drugs are legal. In France, they are not. Drug usage in France is considerably higher than drug usage in the Netherlands. The deciding factor is probably more closely related to the quality of life than to whether or not the drugs are legal.

In addition, no matter what we do, there will always be some portion of the population which is addicted to a substance. Alcohol was one of mankinds earliest inventions. It is in our nature to seek recreation, and enjoy the feelings that (some) drugs create. The fact that drugs are legal or illegal will not alter this.

So, I say let's legalize drugs, go for the tax money, and stop making people who use drugs feel like criminals. If we bring drug addiction into the light of day, we'll be better able ot deal with it, and help those who want help. We'll be able to tax a trade which is recreational, and perhaps save on the amount of money which is taken out our paychecks.