This is the term for Nitrous Oxide, or laughing gas, used to get high. It is usually administered by taking the small metal capsules used in the old fashioned whip cream canisters and piercing the top with a 'cracker' (paraphenelia used for this exact purpose as no one making sundaes uses a cracker) and allowing all the gas to fill a large balloon. The balloon gas is then inhaled and held in until the user starts to feel a high from the gas. This high only lasts only lasts for a minute or less but leaves you wanting more- Hence the association with Crack. Other than killing braincells its fairly harmless (don't take this as gospel) so its more for lightweight hippies instead of serious drug addicts. In addition, its become very popular in deadhead type gatherings and is associated with Hippies in that way too.
Hippy Bob bought four cases of nitrous to sell at the rainbow gathering in Ocala so that he would have gas money to drive his VW bus back to upstate New York.
Bob wanted to get high but had a drug test coming up for work so he sat and did hippie crack all night