Of Mice and Men and Animal Farm are both about characters trying to survive in an unfair system. In Of Mice and Men, George and Lennie are migrant farm workers with no power. In Animal Farm, none of the animals except the pigs have power. In each story, there is a hard worker (Lennie and Boxer) who has incredible brute strength, but not much intelligence.
In each case, this character is taken advantage of by the system. Boxer is sent to the knackers when he becomes injured and too old to work, and a terrible fate would have awaited Lennie if George had not shot him out of mercy.
Despite these similarities, the books also have different themes. Animal Farm focuses on corruption of power, whereas Of Mice and Men mostly concentrates on the effect of powerlessness.
Of course... the two novels are Of MIce and Men and Animal Farm.
Sorry totally forgot to mention them :/