I bought one of these a few years ago for around $120, fully hackable GPS which actually runs a special version of Windows CE. You can get to the Windows interface by holding down the "Menu" and "Volume +" buttons while soft-resetting the GPS. Then there is a hack so that the GPS will automatically boot Windows so that you don't need to manually enter the secret key combination. From Windows CE, you can install any software (video player, games, even TomTom) that you want onto the internal flash (typically 1GB, though some of the units have 2GB) or onto an SD card using the SD slot.
On mine, the only thing I did was change the UI and download an English female voice to replace the English male voice. There are some great UIs on the internet which add functionality to the stock software, or you can just install TomTom instead.