This is a non-exhaustive list of commonly believed myths, and DrexelNORMLs rebuttals.

Myth: Marijuana is a gateway drug and causes people to experiment with and eventually become addicted to harder drugs.
Truth: This is completely false. The statistics used in their argument indicate that people who use harder drugs started with tobacco or alcohol first, or marijuana. However, the vast majority of marijuana users have never touched the so-called harder drugs. The reality is that because of the black market, people who buy marijuana will most likely have access to harder drugs simply because if you know where to buy pot, it’s not a big jump to find someone who sells whatever else you want. From the DrugPolicyAlliance:

Marijuana does not cause people to use hard drugs. What the gateway theory presents as a causal explanation is a statistic association between common and uncommon drugs, an association that changes over time as different drugs increase and decrease in prevalence. Marijuana is the most popular illegal drug in the United States today. Therefore, people who have used less popular drugs such as heroin, cocaine, and LSD, are likely to have also used marijuana. Most marijuana users never use any other illegal drug. Indeed, for the large majority of people, marijuana is a terminus rather than a gateway drug.

Also, from the U.S. Department of Health and Human Services, National Household Survey on Drug Abuse: Main Findings 1990:

As many propogandists will tell you, marijuana use will lead to the use of other, harder substances. More specifically, the argument is, "Rarely do we meet a hard drug user that didn't start out on marijuana." This type of reverse-logic is misleading, however, and as useful as saying, "Rarely do we meet a motorcycle rider who didn't start out on a bicycle." Is this to say that anyone who rides a bicycle will eventually graduate to motorcycling? The fact is, most marijuana users will not move onto harder drugs, despite the fact that users of harder drugs have tried marijuana. According to a survey of drug abuse conducted by the U.S. Department of Health and Human Services, "As marijuana use increased in the 1960s and 1970s, heroin use declined. And, when marijuana use declined in the 1980s, heroin use remained fairly stable." In short, the gateway theory lacks credibility and should be put to rest.

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Myth: Drug dealers lace their marijuana with cocaine or heroin to get you addicted so you buy those.
Truth: Drug dealers have no reason to lace their marijuana with harder drugs, for the simple reason that it’s not profitable. It would cost the dealer more money to lace it than they would receive. Drug dealers sell drugs to make money, and it’s not in their monetary interests to lace what they sell with even more expensive to buy drugs. Sometimes, however, people do lace their own weed with PCP (aka angel dust, wet), but that’s rare and most of the time the person would at least tell you what’s in it if they happen to share it.

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Myth: Marijuana is addictive.
Truth: There is no evidence that marijuana is physically addictive. There is no dependency, no withdrawal symptoms when use desists, and use of the drug can be stopped almost on a whim if need be. While no physical addiction occurs, people can be mentally addicted to it; meaning it becomes a habit. Habits can be broken pretty easily. For a comparison of addictive substances, please visit http://drugwarfacts.com/addictiv.htm. While the government claims that rising treatment rates means that marijuana is an extremely addictive substance,

... Sixty percent of teenagers in drug treatment programs are there because of marijuana. - source

in truth 58.1% of all marijuana referrals to treatment were from the criminal justice system. Only 16.6% of marijuana referrals to treatment were from individuals (that includes parents), which is where you would expect a large figure if, in fact, marijuana addiction (particularly in youths) was a real problem. That clearly shows that treatment numbers are a more a function of referral rather than any actual problems with marijuana use. This is probably because people who have the option of going to rehabilitation rather than jail would rationally choose rehab. (Source)

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Myth: The War on Drugs keeps your children safe.
Truth: The existence of the War on Drugs creates an artificial black market which, because there is no regulation, has no qualms about selling to your children. The simple fact is that if they were legal, drugs would be at least as difficult for children to get as alcohol and tobacco are now. Legalization and regulation, so that responsible adults can purchase and use without repercussion, would encourage legitimate business owners not to sell to people under a certain age, under penalty of losing their business and livelihood. This would ensure that fewer children have access to these drugs. Instead, those conditions do not exist, and kids can get pretty much anything they want easier than they can get tobacco or alcohol. Not only that, but if a college student gets caught with drugs, even a small amount, they can have their Federal college funds taken from them. Until 2006, students with previous non-violent drug convictions were ineligable for financial aid; after efforts to change this by CHEAR, only while in college can a student lose his aid if convicted. This affects poor families who are counting on scholarships and grants the most, thus adding to the stereotype.

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Myth: The War on Drugs is making progress and will be won soon.
Truth: There will always be demand for drugs. That simple fact alone reveals that the war on drugs will never be over until we declare it over, but it will never be won. The government busts suppliers all the time, and even as soon as that happens, another supplier takes his place, raises prices, and makes even more money than his predecessor, because of the demand that still exists. As long as demand exists, suppliers will exist to sell. That’s a simple economical fact. Legalization and regulation is the logical choice.

"Black markets will always be with us. But they will recede in importance when the public morality is consistent with our private one. The underground is a good measure of the progress and the health of nations. When much is wrong, much needs to be hidden." - Eric Schlosser (Source)

"In the latter case of a black market for goods which are simply unavailable through legal channels, black markets will thrive if consumer demand nonetheless continues. In the case of the legal prohibition of a product that large segments of the society view as harmless in spite of its legal status, such as under alcohol prohibition in the United States, the black market will prosper, and the black marketeers often reinvest profits in a widely diversified array of illegal activity well beyond the original "harmless" item." (Source)

According to http://www.usdoj.gov/ndic/pubs11/12620/marijuana.htm,

"Demand is higher for marijuana than for any other illicit drug, and the constancy of this demand over time has ensured marijuana's ready availability and profitability. No less than 75 percent of illicit drug users in the United States aged 12 or older--an estimated 14.6 million persons--reported current use of marijuana in 2003."

and

"Past year marijuana use overall is relatively stable, generally showing modest, albeit not always significant, downward trends across most age groups. Conversely, the consequences of marijuana use seen in ED mentions and treatment admissions show no significant changes from year to year yet continue to rise steadily."(Source)

These statistics and information infer that demand will never fully go away. This means that winning the War on Marijuana is completely impossible, and instead becomes a War on Marijuana users. The goverment has to re-evaluate its decision to persecute and prosecute its otherwise law abiding citizens, or there will be marijuana users in jail indefinately. The saner choice is legalization and regulation, which would effectively erradicate the black market.

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Myth: People who smoke marijuana are evil deadbeats of society
Truth: People who smoke marijuana are just like you and I: regular people trying to make their way in life. Usage ranges from successful businessmen who hide their use from their peers for fear of professional repercussions, to soccer moms who just want to relax after a hard days work. People can use marijuana responsibly and still lead a normal, fulfilling life in the same way that someone who only drinks alcohol on special occasions does. The government propaganda machine would have you believe that marijuana users are "below" you, when in reality marijuana users are all around you, and probably closer to you than you think.