The Andrew File System is the network file system used by the Computer Systems Lab. It is a networked file system with a global namespace, and is in use among many universities and companies.
The CSL AFS servers and clients all run the OpenAFS implementation, but there also exist two others: Arla and IBM/Transarc. The IBM/Transarc implementation is an old version, back from when AFS was being developed by IBM. It is no longer maintained, but IBM open sourced the project when they decided to no longer maintain it, and that developed into the OpenAFS project. Arla was developed while IBM's AFS was not Open Source, in order to provide an Open Source implementation. The client is very functional today and is actively maintained, but the server side is not considered finished yet, and is not widely used. However, Arla's client is compatible (mostly) with OpenAFS servers, so the client has seen widespread use. Although the OpenAFS client is probably more popular in general, Arla can be run on several platforms that the OpenAFS client has issues with (such as the BSDs), and so it has achieved popularity with use on those platforms.