Final answer:
The server that once enabled multiple Mac computers to collaborate on a single task is Xgrid. However, Xgrid has been discontinued since the release of OS X Mountain Lion, requiring users to seek modern alternatives for distributed computing on macOS.
Step-by-step explanation:
The server on an Apple Mac OS X/macOS that enables multiple computers to work together on a single task is called Xgrid. Xgrid was designed by Apple Inc. to provide a simple and efficient way to distribute computing across several machines for tasks such as scientific computing and processing large sets of data. Although Xgrid has been discontinued since OS X Mountain Lion, it used to allow Mac users to create a computing cluster and contribute their processing power towards a common computational task.
Modern alternatives to Xgrid in macOS might include third-party solutions or adapting other distributed computing frameworks compatible with UNIX-like systems, which macOS is fundamentally based on.