Any Volkswagen bus of the 1960' and 1970's. Although most VW Buses that are driven by hippies nowadays are of the 1968 to 1979 Bay Window body style. The 1950 to 1967 Split window VW Bus however is the original mode of transportation for transporting at least 7 tie-dies.
I wouldn't consider a VW Vanagon from the 80's a hippie bus. Although, these day it might be the only option for some hippies looking for a Volkswagen van.
Can also be used to discribe the crazy, psychedelic painted school buses used to transport large amounts of hippie. Buses from the 40's through the 60's were typically used in the 70's. The Further Bus for example. However, hippie bus is most used for discribing VWs.
"Check out the VW hippie bus, they must be going down to Yasgur's Farm. Lets catch a ride with them"
VW Buses forever, Peace everybody!!!