The 4093 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals. The hysteresis is about 0.5V with a 4.5V supply and almost 2V with a 9V supply.
4001 quad 2-input NOR
4011 quad 2-input NAND
4030 quad 2-input EX-OR (now obsolete)
4070 quad 2-input EX-OR
4071 quad 2-input OR
4077 quad 2-input EX-NOR
4081 quad 2-input AND
4093 quad 2-input NAND with Schmitt trigger inputs