Ultransonic parts cleaning is a relatively new thing, but it works great. I bought one big enough to just about hold an engine carter half:
It heats the fluid (22 liters) to 50 degrees and creates micro cavities in the fluid through the ultrasonic action. The result, through the cleaning agent, is a strong degreasing. Through experience I found that the worst grease and dirt needs to be rinsed off and removed or the cleaning fluid won’t last very long and the drain of the cleaner will get clogged.
Here’s an example of a part being cleaned: