One of the most well-known secret key algorithms is the US Data Encryption Standard (DES). It was published in 1977. In the past the US government encouraged users to adopt this as the main or only secret key algorithm in use. In 1998, it was shown that a dedicated hacker with moderate computer power can crack it (see EFF DES cracker project), but it is often considered secure enough for some trivial uses. It is to be replaced as a US standard by the Advanced Encryption Standard (AES).
DES uses a fixed key length of 64 bits. Of these, 8 are used for parity, so in effect the key is 56 bits long.