MDMA
MDMA, commonly known as "ecstasy", is a psychoactive drug of the phenethylamine and amphetamine classes of drugs. It produces stimulant, mild psychedelic, and entactogenic effects. Its common effects include: euphoria, mild psychedelia, physical and mental stimulation, feelings of emotional warmth and empathy, openness, reduced hostility, and negative short-term physical symptoms common with stimulants (bruxism, rapid heart rate, nystagmus etc.).
Since the beginning of rave culture, MDMA use has been linked to it and the culture was branded by the media as a drug-centric culture. MDMA has been a major excuse for the US government to pass bills against raves, for instance the Reducing Americans' Vulnerability to Ecstasy Act (commonly known as the RAVE Act). To the present day, raves are often portrayed as drug filled parties by the media mainly due to MDMA.
The likely reason that MDMA is associated with raves more than other drugs is because of the role it had in building the rave culture. Colorful clothing, glow sticks, menthol inhalers, pacifiers, and other rave accessories are either meant to enhance the MDMA high or are the indirect result of its use, often relating either to its mild psychedelic effect (i.e. stimulating colors to complement the psychedelic sensory enhancement) or to ameliorating its negative side effects (i.e. pacifiers to prevent teeth grinding and bottled water to prevent dehydration).