Tomu is a super cheap Open Source Hardware 24 MHz ARM computer with 8 KB of RAM and 64 KB of ROM that fits into your USB connector! Of course I had to put Commodore 64 BASIC on it, which can be accessed through the USB-Serial port exposed by the device.
It can be found in the cbmbasic subdirectory of my fork of the tomu-quickstart project.
Update: It has been merged into the official tomu-quickstart repository!
Use a terminal emulator to connect to the virtual serial port and hack away!