Rebar Safety Caps
Cut rebar is very sharp so building code requires these caps.
Made of high impact plastic with a metal plate embedded in them.
Need to be OSHA compliant to guard against impalement..
Jackson Safety OSHA #3-7 from Sharpe Safety Supply
About $1.15 each with no min quantity and $11 shipping.
Some people say that when the rebar is horizontal you can get away with lower cost safety caps as there is less danger of falling on the protruding rebar. Personally I think it's best to always use OSHA compliant, but the less expensive type are available from Lowes for $10.98 for 25.