Online laboratories have been widely used in education, research, and industrial applications. For online laboratories, various architectures have been constructed to provide good user experience and powerful capabilities for online experimentation, in which the controller is a crucial part to connect the server with a controlled test rig. Regarding the physical nature of the controllers and test rigs used in online laboratories, a specific control process can be called a simulation, hardware-in-the-loop, rapid control prototyping, or physical control. This article presents an overview of the most commonly used controllers in current online laboratories, focusing on the functionalities, use purposes, and types of different controllers, as well as their role in online laboratories for teaching and learning. In particular, the low-cost PC controller used in the Networked Control System Laboratory, which provides both remote and virtual experiments, is introduced. The limitations and potential development directions of online laboratories are identified.