Use a MIDI keyboard without a PC?

I have a full size MIDI keyboard, and recently got rid of an electric piano to save space. I can still practice on the MIDI keyboard, but I hate having to power up my PC every time, and be locked to it. Is there anything like a self contained sound module with a built in speaker? All the ones I've seen have no speaker, and also have way more features than I need. The PC does all that stuff now. All I want is something I can plug into the keyboard and get something resembling piano sounds out. Surely someone has invented a simple little box that can stop a MIDI keyboard from being a totally useless device without a bunch of supporting equipment...Or do the people making these things really want us all to buy 2 keyboards off them?