Rivest ciphers
Ron Rivest, an MIT professor and one of the founders of RSA Security, has devised a number of ciphers for that company, either alone or with collaborators. These designs are designated as RCn. Officially, "RC" stands for Rivest Cipher; it may also be taken as "Ron's Code".
RC2
RC4
RC4 is a very widely deployed stream cipher.
RC5
RC5 is a block cipher with 64-bit blocks, one of the DES generation of block ciphers. It was the first well-known cipher to make extensive use of data-dependent rotations to achieve nonlinearity. It is a Feistel cipher.
There is an RFC giving an RC5 specification for Internet use; see external links.
Its descendant RC6, also using data-dependent rotations, was an AES finalist. RSA Security have a page describing both ciphers; see external links.
RC6
RC6 is a block cipher that was a finalist in the AES competition. Like all AES candidates, it uses 128-bit blocks and supports key sizes of 128, 192 or 256 bits.
Like RC5, RC6 made extensive use of data-dependent rotations. RSA Security have a page describing both ciphers; see external links.
RC6 is the only one of the five finalists which does not have a completely open license; it is still proprietary to RSA Security.