ACM's primary historical competitor has been the IEEE Computer Society, which is the largest subgroup of the Institute of Electrical and Electronics Engineers. The IEEE focuses more on hardware and standardization issues than theoretical computer science, but there is considerable overlap with ACM's agenda. They occasionally cooperate on projects like developing computing curricula.
There is also a mounting challenge to the ACM's publication practices coming from the open access movement. Some authors see a centralized peer-review process as less relevant and publish on their home pages or on unreviewed sites like arXiv. Other organizations have sprung up which do their peer review entirely free and online, such as Journal of Artificial Intelligence Research (JAIR), Journal of Machine Learning Research (JMLR) and the Journal of Research and Practice in Information Technology.