Amcas is an organization to which you send incrementally larger sums of money for virtually the same amount of actual service, given that all the information they deal with is computerized (after transcript verification). Amcas could not survive in a free market in its current form, but given that it has no competition, it is an interesting economic case study.
Otherwise, yeah, it stands for American Medical College Application Service, and is provided by the Association of American Medical Colleges.