A free bar is a bar that gives away drinks free to anyone they want, and don't accept anything in trade. They accept gifts, especially ones that relate to their operation, but none is required in order to get drinks. Free bars are pretty rare, but there were some during Burning Man 2002 - Safari Social Club to name one. They exist in contrast with barter bars, and regular bars where you have to pay.

Free bars inspire amazing feelings of gratitude, which often results in them getting gifts of more liquor, which allows them to continue operation. Also, on the bar there are usually various gifts, including food that anyone can eat. They are also great fun, because anyone you see, you can just grab and make them come over and get a free drink. Free bars usually attract great crowds of people, who are soon drunk milling around, grabbing more people and liquor.

Free bars last until they run out of liquor. Sometimes they slow down the liquor output in order to remain in operation longer.
Free bars can only occur when there is a feeling of goodwill among everyone present, which is why they are so rare in semi-public events, but common among small groups of friends.