Pirate Battle at Treasure Island (TI). This was the best free show in Las Vegas, hands down. They later ruined the show by trading pirates for scantily clad women who sang and danced around, called 'Sirens of TI'. Nothing compared to the original.
Masquerade Show in the Sky at the Rio Hotel. Full-feathered dancers mimicking the Brazilian Carnival performed inside in the middle gambling area if the Rio for an elaborate, expensive free show. See a panorama of the show. The show unfortunately became too 'adult'. While that show is now closed, there is still free live music occuring in the same room.
AirPlay show at Tropicana. Performers did acrobatics above the main casino floor for a free show that lasted many years until its closure.