a hypothetical megastructure, based on the Dyson sphere, of immense computational capacity. It is an example of a Class B stellar engine, employing the entire energy output of a star to drive computer systems.
This is the type of machine that pushes the limits of computation to the max. Oh wow, that seems great! I'm sure we could all use a Matrioshka Brain in each and every household. I'd use it to match my socks up after they came out of the laundry. Oh wait, it doesn't do that; all's it does is think, and it takes the power of a star to do that. It's a NATO device - no action, talk only. The Jupiter Brain is not much better; it takes too much computronium anyway.
I've got an idea for something that will be really useful, I'll call it the Matrioshka Laborer. This is a device that can do anything, instead of think anything. I think that would be more useful, but what do I know? I haven't run the install on my Matrioshka Brain yet.