The notion of swarm intelligence was introduced to describe the decentralized and self-organized behaviors of groups of animals. Repellents are biologically active things, such as predators, which repel individuals of the swarm. As a consequence, attractants and repellents stimulate the directed movement of swarms towards and away from the stimulus, respectively. The methodological framework of studying swarm intelligence is represented by unconventional computing, robotics, and cognitive science. Some computational tasks which are being solved by swarms effectively such as transporting and scheduling are studied in depth. Nevertheless, there are many sophisticated tasks solved by swarms daily which are little known in computer science, yet. The fact that swarms prefer to follow certain stable patterns can be used in constructing bio-inspired unconventional computers on swarms: computers on ant colonies, computers on Physarum polycephalum (a multinucleated plasmodium), etc.