A popular alternative to eddy brakes is the water brake, in particular for those testing engines at the crank. The braking is controlled by adjusting the water flow, which can be done manually or electronically. More water in the brake = more braking. Note that the braking force is not determined directly by the valve opening, it is the fill level that determines the braking force. Water brakes are typically light and have the benefit that the heat generated is only heating up the water, which you can have in a very large container or cool down at low cost, so long runs and very high horsepower can be sustained. They have a unique property in that they can tolerate very high RPMs, so they can be connected directly to the engine crankshaft.
Water brakes can be tricky to control and the setup can be challenging. The water pressure needs to be very constant and quite high (~3-5 bar). For high horsepower setups, water pumps of 4-6kW is normal! The valve also has some special requirements. Ideally we want a linear response, i.e. 50% brake input should produce twice the flow as 25% input. If you use a normal ball valve, this will not work, since a ball valve (and many other valves) have a very abrupt start and nothing much happens after the valve being half opened. The valve is controlled either by a stepper motor or a powerful servo.
Here are two examples of water brake valves. Notice the very small water passage for low valve openings.
See this link on setting up YourDyno with water brakes.