MARS is a block cipher designed by IBM as a candidate for the AES competition; it was chosen as a finalist, but did not win. Like all AES candidates, it uses 128-bit blocks and supports key sizes of 128, 192 or 256 bits.
It uses a variant of the Feistel structure which they call a "type 3 Feistel network"; the 128-bit block is treated as four 32-bit sub-blocks; each round uses one sub-block as input and modifies all of the other three sub-blocks. Like RC6, it uses data-dependent rotations. One 9*32 S-box is used; for some operations it is treated as two 8*32 S-boxes.
The cipher is now freely available. It has a home page.