Is the social and mutual grooming of another individual.
Primates
Primates are generally thought of when when one says the word allogrooming as many species can be observed grooming each other to relax throughout the day. It is seen as a social and hygienic action as the monkeys remove mites and other small insects along with the dirt from their conspecifics coat.
Birds
When birds allogroom, it is actually called allopreening, as preening pertains to the cleaning/rearranging of a birds feathers. Birds allopreen around the head and neck of other birds since an individual cannot easily preen those areas.
Cats
That's right! Even members of the big cat family spend time allogrooming. Though this is relatively rare in the non-social species such as leopards, yet cats of almost any species can be found grooming each other.