There are several lighter than air gasses. Helium is rare, Neon is rarer, and Hydrogen is explosive and has really bad press. Methane is explosive as well. Amm

There are several lighter than air gasses. Helium is rare, Neon is rarer, and Hydrogen is explosive and has really bad press. Methane is explosive as well. Ammonia, Hydrogen flouride, and Diborane are either reactive or toxic. Helium, Hydrogen and hot air are the most popular options, but none of them are as safe or cheap as plain old planes.