Condoms
It's an external birth control measure used by men. It is a barrier worn in by a man over his penis to reduce the chances of pregnancy and any sexually transmitted diseases or STD's.
This method physically blocks the sperms from entering vagina during sex. This measure is inexpensive and widely acclaimed as an effective Contraceptive measure. These are water-proof, durable and elastic.
They are also available for females and are widely known as 'Female Condoms'.